And how often in practice are terms and conditions attempted to be enforced in the first place? No need to challenge them if you can ignore them

If ignoring them is your only option, and challenging them would fail, we would expect to see a lack of challenging them. Which we do.

Unless there's a solid track record of people consistently challenging them and winning, we can assume, based on bayesian priors, that most people cannot.

Which makes sense: court costs money.