Why do you think writers don't have female superheroes dating normal men a whole lot?
1) Crossover potential.
How many female superheroes have their own individual comic book series/tv shows/etc/? Of those, how many sell well enough to not need crossover with more popular comic books series/tv shows/etc.? I would imagine the number is quite small, and so to keep the options open the writers opt to have the female super heroes date other super heroes.
2) Amount of time in the show/space on the comic pages/etc.
For female super heroes who are part of larger teams, or who share time/space/etc. with other heroes, exploring the dynamics of a Super Hero Woman and a Non-Powered/Non-Super Hero Man requires time/space dedicated to it because you can't simply use the age-old shorthand that was developed for Male Superhero-Non-powered/Non-Superhero female (ie: Superman - Lois Lane) over 7+ decades of comics. They have to develop new shorthand to describe this less-common kind of relationship.
3) Ideas of what women look for in relationships
It is a fairly well accepted idea in our culture that women look for equals or superiors for relationships, and that they are looking for fathers for potential children. This idea is held by writers and readers, and influences what readers expect to see and what writers expect readers to expect. (for both male and female readers/writers) Having a non-superhero man in a relationship with a superhero woman does not fit into this dynamic, and so runs right into Point 2 - to include it requires a large amount of time/space to explain why this relationship is "unique" or special. (Whether this idea is accurate is irrelevant for this discussion - I've actually been googling scientific studies on this topic lately, and the results are extremely mixed and politicized.)
4) Pushback from non-creative executives
Executives are looking to make money, and anything that might not make them money is to be avoided when possible - it's a risk. A "non-traditional" (in the comic book sense) relationship is a risk.
5) General laziness from creative types
Writers run into #4, and decide not to push back against it.
6) Consumers
We're less likely to suspend our disbelief when something pushes us out of our comfort zone. We may not immediately dismiss it or hate it, but we are more likely to look for ways to pick it apart. "She said she had to work late in Issue 7, but in Issue 6 she commented that her law firm was cutting back on hours to save money. How is he not seeing through these lies - he's a lawyer too!" or "Why isn't he asking her about the bandages on her legs?" or "Why isn't he asking how she got all those bruises when she was supposed to be at a company retreat in Aspen?" etc.
None are particularly good reasons or excuses for it, mind you,