There are multiple answers, with different degrees of truth
The patterns aren’t (typically) stored long term, something implied about transporter buffers seems to indicate they can hold incredible amounts of data that starts to degrade very quickly. New patterns are taken each time they transport AFAIK.
But, instead maybe that “cell damage” is just part of the details you get when you retain enough pattern detail to include peoples recent memories.
But, instead maybe the actors age in real life and keeping track of making them look perpetually youthful with makeup would be really hard so whatever the excuse is it’s just an excuse.
something implied about transporter buffers seems to indicate they can hold incredible amounts of data that starts to degrade very quickly
Exactly. I always understood the difference between replicators and transporters to be the level of detail in the scan. The replicators don’t need as much detail to make a convincing steak or a cup of tea. So they can store those scans at a much lower resolution and have a full, permanent library.
The transporters need an immense amount of detail to perfectly store your pattern, to avoid messing with your brain chemistry and causing transporter psychosis. It’s too much data to keep on hand for every crew member.