I guess it depends....
If your bench stands out in the middle of the workshop, with both sides in use, then racking resistance longitudinally and crossways would be equally as important. If your bench is against a wall, as I guess 90%+ are, then, the wall can take the cross loads, and strength against horizontal planing loads is the primary consideration. In the latter case it makes sense to prioritise the strength of the front frame, so your suggestion of building the front and back frames and linking them at the ends makes a lot of sense.
That's the (a) theory. In practise, however, there is little difference either way. They're all just M&Ts into the legs, and it doesn't much matter which ones you make first. I guess it is slightly easier to manhandle completed end frames rather than the much bigger front and back frames, and it may just be that which explains the preference for work order you mentioned. I'll take your word on the latter, though, because there is virtually nothing more tedious than watching yet another Youtuber claim to be making "the ultimate bench" (except of course "the ultimate cross-cut sled"), so I don't.