But don't make Mutt users pay for something they won't use. What this argument means is that adding an MTA to Mutt would very likely lead to adding many other features.
More features means more doc, making it harder for new users to find their way around it.
Said documentation must also be written and maintained, leading to an increased workload on maintainers. It is also very likely to become less accurate as a result. The increased amount of code would probably have similar undesirable consequences. Such as more bugs and longer intervals between releases. Adding features is generally costly because complexity increases more than linearly with features.