Profit Max
Now, your suggestion regarding grounding a/c with inoperable T/R, this is the same issue. If the airline knows that more of their a/c will be on the ground because of inoperable T/R, it will have to buy more a/c to be on standby. This costs money. And passengers are not prepared to pay for it.
Or just MAYBE T/Rs could be made more reliable ?
I've heard of too many accidents where they've come into play.