In a YouGov poll conducted between July 25-29, 59 percent of Republicans surveyed said they would prefer a president under the age of 75. Trump celebrated his 78th birthday in June. Forty percent said they had no preference, and just two percent said they would prefer a president over the age of 75.
Because they actively participated in the deification of Trump among their base, so they know that taking Trump off the ballot would split the party (possibly irreparably).
Especially since it’s a near certainty that Trump would just continue his presidential bid (as a write-in independent if need be).