At least half of the blame should be not on them. IMAP doesn't know a thing about what happened to email in last 10 years (few minor exceptions apply).
They're merely implementing their own means to access email in convenient and modern way. Say, IMAP has no labels[1] or contacts[2] (and many other things) - there are proprietary extensions, MS Exchange being the most famous one, but they're... proprietary. And given that proprietary implementations differ, it makes sense to create an unifying wrapper.
And JS apps and web services are just an approach that's trendy today. Personally, I don't like it either, but can't blame for going with the flow. Especially since it's a business (and going against the flow has costs, both in development and adoption), not a private project where one can do things in a way that just feels right, ignoring anything else.
I believe it would absolutely make sense to bundle the "sync engine" within the mail client, though, to make it standalone. But I understand it's done this way just because there was only the engine (as a service, not a library) at the beginning.
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[1] https://nylas.com/docs/#labels
[2] https://nylas.com/docs/#contacts