Really, there is no real problem saying you built something when it is essentially an assembly of parts. It's like when some people say the built a computer, which is also an assembly of parts.
In many industries, the final builder that puts their name on something is actually an assembler.
It is no big deal. The issue is why someone would want to build or assemble their AR.
For me, it comes down to choices and the ability to get into the project in stages. This is especially important when you do not or can not lay out a lot of money all at once.
Additionally, the process is simplified in that the only item going through the FFL is the bare lower receiver. Once you have that, you just get the parts as your budget or time allows.
Personally, I am less interested in building the uppers, but plenty of others do so.
Just be aware of what you are getting into and what can go wrong. You should make sure you have the correct tools and apply them correctly.

