Because that would be overkill and unnecessarily clutters the SQL. More than that, identifier quoting can fail on certain platforms under certain conditions with certain input ... Lastly, it would add quite some code bloat to quote everything because not all SQL goes down 1 execution path.
All these things together with the fact that identifier quoting is considered a workaround led to the current implementation. We discourage identifier quoting, but still make it possible selectively.
We've had the "quote nothing or everything" approach in D1 with a simple switch and it was/is full of problems. Apart from the simple fact that you need to quote everything as soon as you have a single, reserved column or table name anywhere. Thats just overkill.