Assuming you already have a direct drive hub motor, the 'additional battery charging gear' usually weighs nothing. If you want to, you can design it to use the existing hardware. Mostly its a software thing.Whatever the added weight of the additional battery charging gear is
Now if regenerative breaking gear was very lightweight, cheap and reliable you could just about see it as worth having as an option for when you're freewheeling downhill and would have to brake anyway - free energy is free energy after all. But that scenario isn't actually that common on a lot of rides and the fact that basically no ebike manufacturers have thought this worthwhile suggests that it isn't worth it.
Some ebike manufacturers HAVE already implemented it years ago.