I bought a new enclosed car carrier in 1988 with Goodyear Marathon tires. Two years later the sidewalls were cracked and had MAYBE 2500 miles on it. Went to Goodyear dealer and they said they were fine. Explained I was leaving for Dallas TX (from NJ) in a few weeks with about 6000 lbs riding on these cracked tires in August heat

. They replaced all 4 prorated on tread wear at between $25 and $35 per tire.
As for why the spare doesn't crack as fast, keep i n mind it's not being flexed like the tires in use. I have found covering the tires in the off season does help.