Microsoft had 95% of desktop market share and installed IE on everyone's PC, which they claimed was part of the operating system. This essentially killed the browser competition for about a decade and Netscape got crushed.
Now how is this comparable to Google placing G+ in Gmail? Gmail is smaller than Hotmail and Yahoo Mail? They are a distant third.
Did you create a Google profile by accident? https://plus.google.com/settings/ , click "Delete profile and remove associated Google+ features". Voila, done. Your Gmail is pristine and the chip on your shoulder has surely vanished.
Internet Explorer cannot be removed without damaging Windows or affecting other programs which depend on Internet Explorer's files. You also said yourself it is "pre-installed".
You are not required to be a Google+ user in order to use Gmail, and you obviously won't see integrations in Gmail unless you have a Google+ account.
Because Google has a monopoly on non-shitty free email with GMail, in the same way that Microsoft has (had?) a monopoly on hardware-independent operating systems with DOS/Windows. (I wish I was really being sarcastic, you know because there are actually a billion web-based e-mail alternatives besides GMail that actually don't suck... right? Anybody know of any? At this point I'm earnestly inquiring.)
Gmail might be 'the best', but not enough to get a everyone to switch. So, IMO it's a question of where your suck threshold than anything specifically wrong with Hotmail / yahoo.
Because Google became evil. Remember when Microsoft pre-installed IE in windows? It's the same thing when Google placed G+ in gmail.