unless it's going to Mars

Flight qualified isn't the issue. It's hardened testing. ICs and especially CPUs can experience random and bizarre failures. The analog stuff doesn't.

For example, you can experience anything from a bit flip in memory to line drivers experiencing logic inversion.

The hardening testing requires very expensive and lengthy testing with exposure to different types of radiation localized to specific circuits within the IC. Testing then documents the various failure modes.Then coders have to write code which accounts for the very complex matrix of failure conditions as well as self diagnostics if possible. Additionally checksumming of memory after writes and validating after reads kicks in; usually complimented with parity checking, which itself requires vetting. All of which itself requires test validation procedures and complimentary test suites to be coded. Which themselves require validation.

It's all very complex, very expensive, and very tedious. Needless to say, the CPUs available for space tend to be much older units because of less circuitry and few outside of DoD or NASA are willing to foot the bill for hardening testing.

All of which ignores that CPUs built specifically for hardening is custom with low yields. Which dramatically increases costs in of itself.