why cant enamel paint over enamel?

I have two cars both with great condition factory acrylic enamel paint. Both have had body work on the LR quarter repainted, Both had the paint turn flat after 5 years.
Very close inspection the paint has a crazed fine cracked look, just like old laquar does.
I just happened to read that enamel is not allowed to recover with enamel but requires laquar paint type.
Really? I have no paint experience and wanted to know why:
my paint failed ( work done by body shops not me )
and why cant enamel cover enamel?
Thanks, Glen