Most of the time, when contact is usually sporadic or confusing, it's because you're being used.
Yes, people are busy, especially if they have small children, but usually, being busy is an excuse as to why they are not reciprocating, like they know they should be.
It's flaky or confusing because their reason for contacting you doesn't follow-on naturally from you both having a nice time. Instead, it's more dependent on what they feel like they're lacking in life in that moment, which is on a different timetable. Something happens, then they reach out to their tool (you) to fix that feeling, and once it's fixed, you won't hear from them until it's broken again.
They see you as a service, and personally, even though I've seen this in women, I think men are more likely to see people, especially women, as a collection of different services they can call-off when they need it. It's worth noting that just because they want, or you're good at one service though, doesn't mean they appreciate or enjoy spending time with you as a whole person. The same as they wouldn't necessarily feel the need to have dinner with their mechanic, or emotionally support their accountant.
This type of arrangement is acceptable to some people, but you are entirely reasonable to not be ok with it and seek a more reliable, supportive friendship where contact is not to much on their terms.