Your question, and the currently top-ranked answer, seem to have the implicit assumption that in order to execute a MITM attack, the attacker needs to be in the middle of the network path. So if you trust the path, what's the problem?
Sure, you could be a target of state-sponsored surveillance, or a criminal mastermind could go through the work of obtaining a job at an ISP, gaining sufficient seniority, reputation, and trust to gain access to the routers, then attempt to slip in some malicious configuration that intercepts traffic all while circumventing the ISP's internal security measures and hoping none of their peers notices the exploit because that would certainly subject the attacker to investigation and criminal charges. But I think these are relatively unlikely scenarios.
More likely, I think, is an attack by someone who does not have any privileged access to the network infrastructure, but instead someone who is attempting to remain anonymous, attacking you from a cyber cafe while wearing a ski mask.
So, a likely attack isn't someone with physical access to your network path, but rather an attack on the protocols that determine the network path, so the attacker can bring the data to them, where there aren't things like employment records, security cameras, audit logs, or do-gooder coworkers that might report a security breach.
Those protocols include for example BGP and DNS, which (surprise!) neither one was designed with security in mind in the least, and each have a rich history of exploitation.
And while acknowledging I said it was not so important, do keep in mind that you're not trusting just the ISPs that your traffic goes through, but also every ISP that your DNS traffic goes through, and also any ISP peered with any of those other ISPs that would accept a BGP advertisement for an IP address you care about, which is ostensibly a least-privilege set, but in practice ISPs are really bad at this.
So really, the number of people that could decide to intercept your traffic by abusing their privileged access to the network probably number at least 10,000. In fact it's amazing the internet works at all, but the reason why is the same reason most people don't get murdered even though anyone could do it at any time: most people aren't jerks, and jail time is sufficient deterrent for most of the rest.
So then, why does encryption matter? Maybe it's not so important. Despite the claims of "trust us, it's encrypted!" which for some reason appear on the pages of every website attempting to profit from the collection of sensitive or personal information, it's not encryption that you need, but authentication.
HTTPS with encryption but no authentication wouldn't be secure at all, and this is why your browser throws up a full-screen modal warning when it can't authenticate the server but only an easily missed "not secure" by the address when the traffic isn't encrypted.