BECAUSE it got an INCH shorter over time and was pulling away from the housing. It's about $20 and the new seal is not overly tight but just meets the firewall.
The hole(s) opened between the firewall and the seal allow the cabin to suck in engine underwood vapors, like leaking oil dropping on exhaust, etc. You can kludge the holes with heat resistant metal duct tape but it looks like sheet. P/N is 51-71-8+215+915 Seal, Filter Housing. Takes 30 seconds to install.

I noticed this when going to access heater blower motor/fans that have a slight rumble at low speed.