Imagine the very first game in a hard-reset season, and the teams end up looking like the following:
(All listed ranks would be from the previous season)
3400-2250-4670-1310-3300-3750
vs
<500-1560-3800-1800-2200-2860
This can happen because Comp never actually took ANY other game modes’ MMRs into account, even though a lot of people keep saying that they do.
https://clips.twitch.tv/SecretiveHeadstrongWaspOSfrog - JAKE taking a fresh account, where he was playing against GMs and Masters in QP as he hit Lvl 25 on that account
https://us.battle.net/forums/en/overwatch/topic/20760837867#post-3 - Jeff saying that Comp MMR is separate
Also, you could get to Lvl 25 and become Comp-eligible without ever having played QP, so Arcade-only or even Practice vs AI-only.
Now, with that all being said, since everyone is hard-reset, literally the entire playerbase starts at the same point. As a result, sorting the ranks out would take a VERY long time.
Think Seasons 1 and 2 (3-month seasons at the time). Obviously, everyone started at the same place in Season 1 because Season 1. It took the great majority of Season 1 to get some kind of sorting out since matchmaker was also really loose at the time, not to mention even looser grouping rules (Under the SR1-100 format, a 50SR difference between the lowest and highest SRs on the team was allowed, translating to basically a GM-low Silver allowed grouping).
And then came Season 2, where the ranking system was changed from SR1-100 to SR1-5000. What happened in this season was that it was not quite a full-reset, but the ranks were very compressed.
https://clips.twitch.tv/GiftedShortLEDHeyGuys (Mercer said only a little bit, but the ranks actually ended up more than “a little bit” compressed than he expressed)
Took nearly the entirety of Season 2 to sort out the ranks again as a result, and so many games were so bad for everyone. And THAT is why they don’t want to full-reset, ever. You could make the argument that maybe once every 5 seasons to something like once every 2 years would be ok, but not EVERY season, especially when seasons now are only 2 months.