The question is -- why CoS acquire so much buildings without renting the building space out to other companies?

To start with, I am going to describe a faulty business practice that a lot companies, including GM (General Motors), use on many occasions.

When company’s profits enter a negative territory (losses) the right thing to do is to regroup and try to restructure the company in such way that it becomes profitable. However, not all companies follow this road -- some may think that recovery is around the corner and continue operate the old way waiting for the things to change. They expect changes for two reasons: 1. Improvement in country’s economic conditions; 2. Application of better marketing methods.

GM took this approach to extreme and drove themselves to Chapter 11 (bankruptcy) before the USA government bailed them out.

The other companies use more cautious approach by setting a loss benchmark -- when the money losses reach certain level, the company restructuring begins. The company management are kind of sitting on the fence -- they continue the old business practices for a while, and at the same time lay foundation for the restructuring in case the company’s financial health does not improve.

CoS try, and try, and try new marketing techniques but they do not work, the membership is in steep decline. The profits from Bridge courses go down, too. The CoS top brass are hoping that the conditions improve, but at the same time they are preparing for the company restructuring by acquiring real estate in various countries.

At some point the Bridge courses will be bringing so little money that having tax exempt status will be of a very little advantage. At that moment the cult will begin using their real estate holdings as a commercial property. The religious cult will become sort of a financial firm.

This is just my hypothesis, of course. Only the time will tell if it is correct.

Simple. It implies success and power. If they rent, it gives off the public impression they need the rental income. Even if they do.

The cult has always had an arrogant air to the way they do business, and it's often a major foot bullet, but habits are hard to break in the cult, especially when DM knows he can always rely on his mindfucked minions to continually "bring me the money".

They have previously acquired buildings that still have tenants and when they do this they have to pay full property taxes. You might think that's fine because the rental income would cover all that and more. However, when tenants learn that the new owner is ultimately going to turn it into a church and not continue rental contracts then they tend to get out as soon as they find somewhere suitable, which leaves a few hangers on (not enough to pay property taxes). I've not seen any evidence that the cult looks for short term rental customers, or would even want to because most (especially retail business) don't want to be on short term contracts. Most of the buildings they have acquired are empty and stay empty, often for years, before anything is done about renovations. When they are empty they are a real phsyical bricks'n'mortar excuse to reg more money out of people (who were already regged to the hilt to contribute to the purchase) for the cost or partial cost of the renovations. It's like "come on folks, we've got the building now, we're nearly there, just another few millions $". These pending Ideal Orgs also become adverts at the large scientology events such as the IAS anniversary and new year events to encourage more IAS donations. So, in many cases, the mothercult can do quite well with an empty building. The less the mothercult has to fork out for purchase and for renovations then the better for them.
Then when they are done, the mothercult owns it and rents it back to the local scientology church organisation even though they are the ones that largely paid for it.

What the cult somtimes does is purchase more than what they need and then sell off extraneous land/buildings attached to the main property that is the actual proposed Ideal Org building. This happened in Birmingham (UK) with the old Pitmaston House building (search Org Patrol forum and media reports) and the money did not go back into the local org to pay for renovations. If it had then they would have had an Ideal Org there 3 years ago. Instead, according to local email leaks, the local scientology organisation is playing fast and loose with individual member's personal and business finances as well as encouraging abuse of bankruptcy process as they are forced to scrabble for donations. The cult made money from Pitmaston House and its grounds. It will earn interest for the mothercult and if they do have to dip their hands in their pockets to help the local org with the renovations then they'll probably still have come off better on the deal, or at least will have broken even on costs, and with a big property asset to boot.
I think Birmingham Pitmaston house is their best chance because of that land sale and the fact that the building is actually habitable but the rest of the handful of proposed UK Ideal Orgs are just falling apart and one wreck, in Northumbria, is even being sold off as a fire sale.

I am not going to defend my hypothesis because I have no data to support it.
This may be just their arrogance, as Smurf says. Sponge might also be right.
I really do not know for how long they will be keeping those buildings empty -- they might not know that, either. It seems to me they are trying a new approach on the trial-and-error basis. I based my hypothesis on data that comes from other companies, but I cannot guarantee that it applies to CoS.