A friend of mine recently discovered that his house key opens the main door to another friend's apartment building....

With bike locks as with many things, you get what you pay for. And in this case, "pay" means not only money but also weight. Better locks are generally heavier as well as more expensive.

For what it's worth, even if the lock's construction is such that there are only 25 possible key types, it's likely that if a thief is determined enough to try carrying around all the keys it would take to be able to open, say, the four or five most common models with relatively few key types, that thief is more likely to have quicker ways of stealing your bike than to stand there trying keys.

It doesn't mean it's impossible that some opportunist will try their key in your lock, find that it works, and take your bike. But even with a lock like that, the matching key thing doesn't seem like the most likely method of taking your bike. If you're locking up someplace where you think you need to worry about that, you probably should be using a better lock overall anyway.