Making templates that are nice to look at and compatible is hard. Obviously, you can send emails for much less but I have yet to see a WYSIWYG editor as easy to use as Mailchimp's.
They created MJML[2], an email templating language that I've used quite a bit. The WYSIWYG editor for Mailjet leverages it, and creates pretty slick looking and compatible email templates if you don't want to use MJML directly.
[2] https://mjml.io/
That said, you don't have to use Mailjet the service to use MJML the templating language. I've also used their WYSIWYG editor quite a few times to edit templates and export the HTML and MJML files for use elsewhere.