Why did he only start unless an injury happened? What am I missing?
Like most things its probably not any one thing. Could only "really" discuss by sitting together & watching film so I'll ty to quickly address.
1) CWill was clearly not a good G. Either favoritism or coaching incompetence allowed him to start as long as he did. He has much more functional power than CWill. He can stymie most bull rushers. McG was never going to start over Tyler or Peters due to ones draft position & natural strength and one's HOF career when team thought they could get both on the field. However, they quickly learned Peters was only good for limited number of plays.
2) McG isn't a natural on the left side. He clearly stated his preference for the right.
3) When he did replace CWill for a few games in 21, he certainly wasn't great but he got blamed for some things that were really TB. This is the basis for a lot of the criticism on him but it's misplaced.
4) He started the 22 season but got injured in game one (I believe). He missed several games and when back they shifted around some with Tyler & Peters. After that he became the primary player at LG. He & Tyler as well as he & TB developed some nice chemistry by this time. Perhaps he also got more comfortable on the left side. If you think back, most of our successful runs came behind the left side due to these three playing well together. McG would often get multiple blocks on the same play. Often the linemen furthest downfield at the end of the play.
5) I don't know where he will peak but if I was a team looking for a legit starter and had to choose paying what he got between a guy with only about a season's worth of playing time who appears to be ascending and the same for a known, trusted commodity but who is 8-9 years into their career, I'm taking the younger guy and hoping what I saw last year was his floor and that I have a bargain if he keeps developing.