Just so happens I've been working with email for twenty years. I've heard of this Google email thing, and while it certainly exists, it's not an isolated case. Mail server admins are empowered to handle incoming mail in very many limiting ways, whether that be rate limiting, or spam filtering, or message size, or lots of other things.
While there are general standards for these kinds of limits, they all exist at essentially every receiving mail server - and for good reason. You have to implement limits, or it becomes elementary for your mail server to be attacked and endangered.
Because Google has a large stake in email, they are a large target for such attacks. It stands to reason that they would need to have strict limits in order to reduce their exposure. But again, all mail servers have various limits applied, and we're still using SMTP.
Oh, and I forgot to mention: if Google wanted email senders to adhere to their limits, they would make those limits public. They don't, because doing so would just tell malicious senders how to work around those limits.
NotTheOnlyGamer