There are many reasons to not do that. For example, the basic block reward was initially 50 BTC, and if people would reset their extraNonces in every block, those wallets could remain unlinked to this day. If you have one huge wallet, and you always use it for all payments, then even if you change your address in every transaction, you can still be traced by your amount.

Also, if someone wants to buy something, it is generally a bad idea to always use your single coin, with your all lifetime savings. Because it is like going to the shop, buying one ice cream, and using a single gold bar, that is worth more than one house. You usually don't want to spend $10, and say out loud "I did it from my account, where I have one million dollars".

Of course, there are ways to handle that correctly, for example CoinJoin. And you can as well pretend, that this "millionaire" is not you, but some exchange. Or you can have a sidechain, that always uses a single on-chain output, and is moved from one address to another every sometimes. So, there are ways to safely use a single coin, but they may not be instantly obvious, and it is too easy to reveal to the rest of the world, that you are very wealthy, so the whole OPSEC can be harder in this case.