Can anyone share why they expect net neutrality from private corporations?
Because as a consumer, that's what I'm paying for—the internet. Not the part of the internet that paid their protection money.
This isn't a problem if Capitalist Theory works the way it's supposed to. I can simply switch from Verizon to Sprint.
Unfortunately, as you say, the problem is monopolist and anti-competitive practices. I can't simply switch my mobile carrier because I'm locked into a multi-year contract. I can't simply switch my landline provider because there are only two in my area and neither provides true internet.
Like many economic policies, it comes down to impractical purist Capitalism. Pure Capitalism simply doesn't work. I can't think of any pure theory that does. Ideology must give way to pragmatism, and purist Capitalism must be tempered with the minimum necessary regulation to protect consumers against definitively sociopathic corporations.
For Common Carriers, that means anti-discrimination laws. For ISPs, it means so-called "net neutrality."