If Quantum MIGHT become a threat to Bitcoin and it IS possible to create an algorithm resistant to Quantum Computing, is there a reason we do not make Bitcoin stronger yet?
The main reason is that there is no consensus how to switch and to what algorithms. To introduce a new soft-fork, someone has to make some proposal, get it discussed, create a BIP for that, and go through the same process of soft-forking as changes like Segwit and Taproot did. It's not something that will be introduced tomorrow, because some people think it is a good idea. It's something that will take a few years at least. But you can start that process if you have some ideas how to switch and into what exactly we should switch.
What I described above may be acceptable when it comes to block headers, but we also have other hashes. And in that case, we would need re-hashing everything that uses SHA-256. Here comes the first question: what function should be used in that re-hashing? SHA-3? A combination of some new function and SHA-256? Also, the current solution will be less efficient that it may be in the future, because if it is publicly known how to create any preimage for SHA-256, then you can use that knowlegde and require such solution in every hash. As I mentioned, you can replace 64-round SHA-256 with 16-round SHA-256 and try to protect it somehow, for example with SHA-3. Then you will see, what can be attacked, how to attack, and you can start designing soft-fork to some new hash function; it is not that obvious, how to make it "soft", that's the lack of proposals and the lack of consensus about it, someone has to build it.
Many computer systems are based on unsolved mathematical problems. Hash functions we use today have some properties that makes them strong. If they will ever be broken, we will have one more solved mathematical puzzle and at least one more open mathematical question. The new hash function will be probably designed, based on such attacks, so it is hard to know the weakness upfront, because you don't know what needs to be protected.
Just be the change you want to see and propose something. I described above how any new hash function could be introduced in block headers, but that's only the small part of the solution (also it has a nice property that if you can reach SHA-256 with all zeroes, then it is the same as putting your new hash function directly in the same field, so it is kind of "gradual activation" with backward-compatibility, similar to how we have new transaction hashes for Segwit). There are many things to design if you seriously think about it, and the lack of detailed and well-discussed proposal is what stops us from switching.
Let alone, it'd make the system less efficient.
It is possible to build some network with re-hashed blockchain that will switch only after seeing a proof of breaking SHA-256. The Script is enough to describe both collision attack and preimage attack, also second preimage attack can be handled. So, technically you can protect yourself and convince people to use your software (having some working code covered with tests and running on some test network is the bare minimum if you want to ever see that on mainnet).