Well I think the better question is how can it take anyone more than a day or two to gear up on non-seasons even as a casual player (assuming you aren't a brand new player.) There was jack shit to do with crafting mats until 2.3 so if you've been playing even a little over the past year and a half you should have stupid amounts sitting in your stash unless you just chose to not pick them up all this time. When 2.3 came out I logged onto my Monk, who just to give you an idea of how old she was, was wearing Raiment with a Jawbreaker. I just cubed some shit and was ready to go in less than 10 minutes. It's only sillier when you consider that the best sets for a lot of classes right now are sets that have been in the game from the start. My Wiz and WD already had Rasha/Zuni sitting in their stash because why wouldn't they? I had all that crap before seasons, GRs, etc. were even a thing.
I just have a hard time believing it takes more than a couple of days to gear up to the current endgame builds on non-seasons even as a casual player, with the exception of brand new players of course.
My Wiz had DelRasha almost ready to go when I came back for 2.3 except for some stuff like FnR. Yes I had plenty of mats but I didn't want to blow all of them in one go crafting rares at the Blacksmith and upgrading them till I got the ancient AW I needed. I'd rather run some rifts and upgrade the rare wands I picked up. Was easier on my resources and was fun upgrading as I went along (which is probably somewhat similar to a season character).
My DH still runs M6 sentries and I had zero UE pieces and since it's more efficient to get these via Blood Shards, it's taking quite a bit of rifting to get the pieces I need for UE Multishot build.
Yes, I could have blown through my mats early to get what I needed but that would have been an inefficient use of an increasingly valuable resource and the process of getting the items I needed via rifts and shards is fun anyway.
It really is not that different from seasons. A lot of season characters are getting pleveled to 70 anyway so it's still the same slog (but longer and far less efficient) to reach end game builds.
So my point is, for the people who don't care about leaderboards, there is very little reason to be playing seasons. Even season exclusive items eventually drop in non seaon anyway and there aren't that many of those.