Because 1) We're almost certain to lose our most athletic OLman, LG Joe Thuney;
2) Our next-most-athletic OLman is LT (as it should be) Isaiah Wynn; however...
3) Bill just re-acquired his SB-winning LT from 2018, Trent Brown; so...
4) Unless there are plans to re-sign Thuney (or a reasonable & cheaper facsimile thereof) AND move Shaq Mason to Center, Onwenu to RG and Brown to RT, then the logical conclusion to draw is that Brown returns to LT (provided he can show that he's still capable of playing well there), Mason & Onwenu stay at RG & RT respectively and as befitting their athletic abilities, and Wynn moves to LG where he also played in college and where his physical profile is better-suited anyway...
Look, if Bill, Carmen & Pop decide that Brown isn't cutting it at LT, then they can always try Big Mike at LG, keep Wynn at LT and move Brown to RT...Not as good as Brown-Wynn-Center-Mason-Onwenu in my opinion, but it's there...
My actual dream starting 5 is Wynn-Thuney-Mason/Andrews-Onwenu-Brown...