Back when sending emails was about 1 cent per email, my company wrote our own campaign email software to save tons of money. Each large email provider has its own rules about sending rates, backing off, soft bounces and hard bounces. Not getting your IP address blocked for breaking a ruleset that you don't know about is the hardest thing. A few year later MailChimp was becoming more popular and their prices where dropping; we switched almost everything over immediately and life was much easier.

This would be true if Mailchimp had a competitive advantage in deliverability, which they don’t. Source: I have evaluated Mailchimp as an ESP several times and selected competitors.

None of the ESPs beat our corporate email system on deliverability BTW. They did make it a lot easier to manage email programs, though.

Edit to add, I’m sure IP reputation management is a challenge if your business model is to allow anyone to sign up for free and start sending. Paying clients don’t care about that, though. Deliverability is something you can buy from lots of people. And for small folks, you’re not going to beat the deliverability of Gmail. It’s table stakes IMO. There’s a reason Mailchimp sells itself as a marketing software platform and not “we deliver emails”.

Their competitive advantage in deliverability is their client list. You can't divine yourself a massive volume of legitimate and engaging traffic. MC had to answer to literally no one, and we got to fire any customer we wanted.