My 2-cents is the simple reason is...money. Rather, there's a huge tech labor shortage in most of the world. It's easy for the countries (U.S./EU) to transport simple or self-contained shells, missiles etc. But another for equipment that requires huge amounts of experienced technical labor. The U.S., as is well known, is having huge problems with enlistments. 20% below quota. I bet it's more like 80% for high tech.

Even if a country wants to give Ukraine F-16, F-18s or Helicopters it must FIRST have their tech people prepare them for use and set them up on arrival, then train the Ukrainians. I believe they simply do not have the manpower to do that.

You probably know more examples than I, of things the UK and US haven't been able to do because they simply didn't have the needed experienced people to work the ships, etc.

No one in the U.S. government is going to say this explicitly. There literally aren't enough people to operate everything we have. Skeleton crews are the norm. There is NO solution for a variety of reasons. War will change that, of course.