Willson Contreras was 30 when he signed his 5 year deal. Murphy was 28 when he signed his six year deal.
A 6 or 7 year deal now would leave Contreras coming out of contract at 33 or 34 having signed away the best earning years of his career for just 15 million AAV. (Realmuto 23 mil. AAV, Salvador Perez 20 mil. AAV, Wilson Contreras 18 mil. AAV)
The Dodgers contract with Smith is unique because he has a guarantee for what is almost certainly the rest of his career, plus its front loaded as he gets 30 mil of the 140 mil up front as a signing bonus.
Like any player, if the Brewers were willing to pay market rate prices they likely could sign Contreras to an extension tomorrow.
However, Wm. Contreras has no logical reason to sign an extension like the proposals above. It would be for less AAV than the best catchers in the game get, and would end at the exact time most catchers experience a drop off in play