Why do people use "yeah" instead of "yes"?

It's just the same reason as why people like to say funner even though they know it's not always the correct thing to say.
I love to use yeah because yes is too formal all the time and it really doesn't express how I feel at that moment.

It's considered informal and friendly, helping to ease social interaction.

Because it sounds good.

because it is kind of cool to speak this way

I say "yes" a lot,no offense but who cares if people say "yeah" it means the exact same thing as "yes"

Less rigid.

Yes! there are people who think Yeah is too informal. But think about the formal substitutes like Okay, Yes, All right... etc. They are too formal& monotonous if used as a 'conversation-binder' all the time !!
ULTIMATELY, it depends on HOW YOU USE it, instead of What you Use. Also keep a tab on Listener's prejudices against the use of 'Yeah'. If he / she has no particular objection, it's all right...!!
I prefer the Use of ‘Yeah’ as it has a greater potential to express a range of emotions by virtue of its greater Vowel Stretch — without sounding presumptive (as in the case of ‘okay’)
The purpose of Communication is to put the Message across in as convenient & Pleasant a manner as possible. That is it !!