If you tell Thunderbird to send it later and then quit the client (turn computer off, whatever) - what do you expect to happen? Probably they would not implement anything other than 'it sends when next online after the send later timestamp', but that probably would catch people out and generate a lot of noise in bug reports.
And you might think you're ok with leaving the client open now, but then some time send later & forget about it or think it's happened, close laptop, and then it's not sent until too late.
Maybe Mozilla's considered and decided it can't deliver an experience it's happy with without a server component. Maybe it thinks a server component might be on the cards as part of some subscription thing, but some other things need to align first. Maybe it hasn't considered it at all.
But it's far from a trivial 'gah why won't they just implement this little isolated thing', imo.