Because even just 0.1%/year of $1 trillion is still $1 billion, or $20k/block. That's a lot more than you could easily raise by donations. And if it's consistent (which tx fees are not), it's probably enough incentive to keep mining moving forwards consistently.

...and that's a key point: sufficent tail emission or demurrage can fix the reorg problem where big miners frequently have incentives to play games reorganizing blocks. Fees alone simply can't do that all the time, as they'll inevitably be inconsistent.