Why doesn't GMAC/GCM encrypt its authentication tag?
Latency, probably. 

$\mathrm{GHASH}_K(m) \oplus \mathrm{AES}_K(n)$ lets you compute both in parallel, whereas encrypting the $\mathrm{GHASH}$ output would add significant latency for short messages. The authors go to great detail about this in Section 3 of the original GCM document.

A similar competing mode at the time, CWC, did indeed encrypt the MAC output but was not as efficient.