Easy to say in hindsight. Before the internet/web existed as anything viable, the only way to have a reliable email service was through companies like AOL, and several of their competitors unless you were a student and your university had its own network/domain. When I went to university, there was no such thing as the internet, we used mainframes on time sharing and acoustic coupler modems at about 12 baud. When I started with Shaw, email was one of the main services people used it for, the internet was mainly text. When I created my first email account with Shaw (you could only have one back then) it was the main purpose for having Shaw. I still have that email address, in fact, it is to this day my main email address. Over time Shaw offered access to more email addresses and charged more for their services, as broadband speeds increased. Non-internet provider email services came and went like the seasons, so using your ISP made the best sense, as a pay service to make sure your email addresses would be retained as long as you maintained your account with that company. Besides my Shaw email accounts, I've had dozens of other via freenet, rocketmail, Telus, Yahoo, half a dozen different Microsoft domains, as they kept changing their domain names, etc. There are still people who have attempted to contact me via my freenet address, and its been decades since I used it. The ones that have remained consistent are my Shaw accounts. At that point, they were pretty entrenched. That is why many of the people still using the Shaw domain are seniors.
I don't disagree that there are advantages to having email services separate from ones ISP. For one thing, it makes it easy to change providers. Shaw has "kept me hostage" as a result of my emails accounts. Quite honestly, Telus, on numerous occasions, has offered me much better services at much lower costs, even just recently, and the only thing that kept me with Shaw was my email accounts. However, there was a time when it appeared that even the larger free email providers (like Microsoft or Google/Alphabet) would eventually start charging or they would cancel the services, just as Yahoo did with Yahoo groups. Gmail is a personal information vacuum cleaner, so while I have numerous gmail accounts, I only use them for my several Android devices so I can access the Play store.
It's easy to say "People shouldn't have an email address from their internet providers" in recent years, because there are stable email services now that aren't likely to disappear (like Outlook, etc) but my email addresses were decades old by the time I felt I could trust Microsoft not to pull the plug. Also, I prefer using my own email client rather than their navigation and layout. BTW, have you ever had to deal with an issue with your email account with a free email provider like Microsoft or Google, good luck, pretty frustrating when things get scrambled.
So, while today I don't disagree with your sentiment, by the time many of us had access to a reasonable reliable (and free) email option, we were heavily invested in what our ISPs included in their packages. As others have stated here, I also have a good 1000 places and individuals who use my email addresses to connect with me, many of whom I honestly don't even remember who has been given them. Also, some of my email addresses are on websites I don't control, which I gave them permission to do, because I offer free tech support in specific areas. Lastly, I have some very clever email addresses on the Shaw domain which I could never get these days on any public domain, they'd be long gone. Believe me, I've tried.
I'd like to see the same features which cell service now has, of portable phone numbers, so regardless of the carrier you use, your cell number can go with you. That change only occurred ??5-10 years ago?? Before that, your mobile provider owned your phone number and if you left them for another, you needed to get a new cell number. A similar system could be developed to transfer email addresses, or at least to maintain servers which allowed for email to be diverted and forwarded seamlessly for the outside world.
