Because google is an advertising company, and makes money by spying on users.

Google being found to scan emails received in gmail and advertise on that basis is why amazon receipt/invoice emails no longer include details inline.

GMail is not a free product. You are paying google with incredible amounts of your "private" information.

Because Google chose to implement an open protocol and to operate their email service as part of the broader email network. Apple chose a different path with their iMessage system, and that's fine too.

Because email is a decentralized network! iMessage isn't! iMessage isn't email! It's nothing like email! Also if an email provider wanted to charge other email providers to federate with them, they can. No one would pay it because there's too much competition for email services - it's hard to compete with free - but nothing is stopping anyone from charging for e-mail peering the same way ISPs do for regular internet currently.

Why Gmail users don't have to pay to send e-mail to your mail host?

Ok, should Sony be _required_ to support playing playstation games on an Xbox if people are willing to pay?

It would have to be free, otherwise they become customers. The current option is to pay for an apple device.