Why does a big company only look at your diploma and a startup only look at your github ?

Cause big companies are old and slow and that's how it used to be done

If you get a few applications per month you can afford to look closely at each individual application, if you get hundreds or even thousands each month you have to filter them first and things like degrees and experience are easy to filter on.

FWIW I work for a large company, not a startup, and GitHub is far more important for my screening than education.

i work for a large company without a diploma / degree.

not all companies are that slow and behind the times.

Because start ups aren't at a corporate stage yet.

Here in Brazil many IT companies work for government agencies/companies and they always ask for graduated or certified employees in the contracts...

Honestly though what if you don't use github but some other git platform?

Because a diploma proofs that you are smart enough to complete 4 years of hard exams and so.

Where your little side projects proof that you can follow some tutorials and maybe adapt them a little.

@Double-A
I'd argue with that.
Diploma proves that you can cram stuff that you'll forget 1 month later
Side projects prove that you are genuinely interested in the field and capable of developing yourself