Why isn't MU shown on Runner identities?
Without answering for the designers, I'd say that it's simply because the Runner's MU is a basic game rule. It's not something that typically changes with Runners, but it's a resource to use.

Similarly, the Runner identities don't tell you how many cards are in your starting hand, or how much money you start with, but that doesn't prevent there from being a Runner identity that does these things differently.

Because, in most cases, the Runner identity doesn't play around with MU (compare to Link; most Runners have 0 but a lot have 1), it's just not useful to spend card space and player attention on the Runner's starting MU. It would be more distracting to see starting MU on the card, as if it were an important factor in how the Runner works, even though [almost] all of them are identical in that regard.