Two aspects: The number of utxos the system can support is not infinite; to verify each of the 1000s of transactions in each block, you have to check all the inputs are not already spent, which means you need fast access to *all* the utxos that currently exist. At currently 1-200M utxos, this is already quite a task. The earth's population is more like 8 billion.

But the other argument/aspect is economic : if everyone owned utxos, it could means that funding them and spending them requires block space in huge competition... costs of txs would spike way beyond the reach of the average person.