How does a base layer cool?
As has already been mentioned, it keeps your sweat on your skin where it's evaporation cools your body. Thus such a baselayer must be skin-tight.

Second, imagine a fan blowing air over a large bowl of water. Water is evaporating off the surface, right? Replace the bowl with a wet towel of equal diameter. From which will water evaporate more quickly. The towel, right? Because there's more surface area per measured area.

So a jersey would work just fine, if it were also skin tight, as tight as a UA garment. Perfectly fitted skinsuits are that tight. However, most of us aren't about to wear a jersey that tight, even if we wanted to bother having it tailored for us.

Anyway, that's my reasoning for deciding to try out the concept. $30 for the UA HeatGear LS shirt on Amazon.