ECC has only 10-15% more transistor count. So you're only making one component of the computer 15% more expensive. This should have been a non-brainer, at least before the recent DRAM price hikes.

Also, while computers may not be used much for cosmic rays to be a risk factor, but they're still susceptible to rowhammer-style attacks, which ECC memory makes much harder.

Finally, if you account for the current performance loss due to rowhammer counter-measures, the extra cost of ECC memory is partially offset.