How come you are supposed to tip the same in Texas and California?

In Texas waitresses make like $2-something an hour (tipped employees get paid a lot less in other states) and in California they make regular minimum wage that is required to be the same for tipped or not tipped employees. How come the standard tip is like 15-20% in both states? I would think that you should tip more in Texas because the way that it is set up assumes that those waitresses receive tips to cover their pay. And everything I read online about why we tip at restaurants in the first place has to do with how the servers are underpaid because we are supposed to tip and make up the difference. But this isn't the case in California, so why is the tip standard the same?