Those guys aren't middling OT, they were good until they became injury busts.
Betcon was an injury and character bust. He was good very early on, but would show up 50 pounds overweight to camp every year and missed two full seasons with knee injury. After that he was an unplayable bad RT He played well as a GUARD for the eagles, less so for the chargers. His lost athleticism since the knee issues have relegated him to guard. He is currently a FA.
Willis started out as good, injured his knee in 2023, never recovered, and hasnt really played (and been horrible when he has). Willis has played in 4 games since he first got injured in 2023. That's why he is on a 1 year vet min deal with Chicago to be a backup.
Your point of "why do teams pay OTs mediocre deals" might not apply to Willis or Betcon, but it is a valid question. Teams pay bad OTs money to play for them because they have no other choice. Just because they get paid doesnt mean they are good.
Look at Dan Moore Jr. for the Titans, he was awful for the steelers and he's been awful for Tennesse, they still paid him a ton of money to be awful, because he was slightly less awful than Dennis Daley.
Or Jonah Williams for the Cardinals. He was a massive bust for the bengals and we paid him a bunch of money to be terrible here.