its not as easy as "oh duh just give itch.io buyers the steam keys". devs are limited on the steam keys they can give out (best case scenario is 5000 keys no questions asked, but would likely be less for a small indie dev with only one other [free] game on steam), and anything after that needs to be paid for out of pocket or approved by steam. a bare minimum estimate for how many itch.io buyers there are would be 100k, which is an insane amount of keys to be given out even across an entire games lifespan, let alone any time soon. if you care so much, spend the extra pocket change on a steam copy. if you arent willing to do that, then you dont care enough to deserve a steam key regardless.