I think it ultimately comes down to having to support another platform ads more work and risk then its worth.
For example theyd have to convince people to essentially break androids walled garden which means valve has to make sure everything they offer for android isnt malicous. Then there is the fact that phones are different specs, more testing, potential customer support … Its a whole can of worms, but valve would gain little. There arent many people who want steam on their phones but dont already have it on pc.
Vavle is good at what they do, they have basically cornered the PC and portable PC markets already, i think not mucking around in an already monopolistic mobile market is a pretty wise business decision on their part overall.
Valves strategy of not doing anything new in the last decade while every other publisber shoots themselves in the foot seems to be working pretty well for them.
My uneducated guess is that most of the time spent on any optimization has to go to console ports. With PC you can just tell people to get a better PC, but with Sony and Microsoft they will straight up refuse to sell your game if it doesnt run well enough (see: CP2077 on ps4 or BG3 on Xbox).