I'd imagine that there's a reason why Adams is the only receiver worth a damn to get a 5 year contract this offseason, and no one got a 6. Doesn't seem very "bog standard" to me.
Kupp, DK, Deebo, and Terry all got 3. Godwin and Williams are on 3s as well.
There is a very good reason there was only one contract given to a WR this offseason that was for more than four years, and that was to the best (and dirtiest! Let's not forget that Davante Adams is a dirty cheater) WR in the league, and came from the Raiders, who are currently throwing money at trying to compete in that division. And even Adams's deal is really more of a three-year deal, with very little guaranteed after 2024. And if you try to keep the player performing on your team without guaranteed money, you get the Earl Thomas situation from 2018: a player who doesn't want to be on the field taking injury risks (including the risk that he might get injured in a way that keeps him from making big money for the rest of what could otherwise be productive years for him). And what happened to ETIII (among others) is a big warning to other players that they should really fight hard to avoid playing on contracts without further guarantees.
I believe the reason is that
@Fade's analysis (minus the totally unfounded assertion that every team except the Seahawks gives WRs six-year contracts as S.O.P., which is just plain false/incorrect, as
@Maelstrom787's excellent post shows clearly) ignores risk management for both the team and the player. Yes, it's nice for teams to have players locked into long-term deals when those players play well over those entire deals. But the problem is that the player may
not play well over the life of the contract, and if that happens, then you're stuck with a multi-year albatross, paying eight figures for a player who's not worth it. Teams didn't even use fake six-year deals this offseason, where the extra years are there to allow part of the cap hit to be reduced over the next season or two, via a reduction of what part of the signing bonus is counted in each season. And from the player side, there are good incentives
not to accept contracts with (mostly) non-guaranteed years at the end. Even ignoring that, a player who believes he might play well enough to make a six-year deal a tremendous bargain for his team has major incentives to make a shorter contract and have the chance to make a lot more money a few years down the road.
So it might just be good risk management by both sides that keeps the contracts shorter.