Because they would sell more games in the end. I guess this is exactly what will happen: A Steam Runtime for 32bit x86 games bundeled with FexEmu for M1 and a 32 bit x86 runtime for macOS in general.
I don't think galve trusts apple to do anything different at all - apple maintains much more stringent control of their os and what software can be run on it, and also asks for platform fees.
Valve wants none of that. Apple will develop their own game support and it will be for an apple gaming platform that they'll take 30% for.