I think the Mason trade is the intersection of multiple things, primarily age and value relative to salary/performance.
He's getting old(er). 29 years old (by the start of the regular season) isn't ancient, but it's getting up there for an OG in what will be his 8th NFL season.
He hasn't been as good the past few seasons. He was borderline elite from 2016-2018'ish, but has slipped a bit recently. 2020 was a poor season for him, and while 2021 (last season) was better than 2020, I don't think he returned to the level of play he did during the 2016-2018 timeframe. If you're getting 2016-2018 Mason, you have a fantastic player on a steal of a contract; if you're getting 2020 Mason, he's a liability; if you're getting 2021 Mason, he's a good (not great) player on a reasonable contract.
My biggest gripe with this trade is that a 5th seems a bit modest; a 4th or a 3rd would've been seemingly more reasonable, with a 1st or a 2nd being unrealistic IMO. Ultimately, though, a player's market is the most someone else is willing to offer. The lack of better trade packages on the table may speak to the above points about age/on-field performance (particularly the latter).