Understanding Dipole vs Commercially available antennas
Why are so few products available along those lines?


As a general rule, if it's easy to build, doesn't have a huge market share, and every customer will want something a bit different, then it won't be available off the shelf.

This was the case in the 1980's and 1990's for power inductors -- every switching supply design was unique, and inductors are easy to wind.  So you could get every kind of inductor core off the shelf, and just about every moderate-sized technical center (e.g. Portland Oregon) had several coil winding companies that would custom wind you coils -- but the actual selection of off-the-shelf coils designed for switching applications was just not there.  Contrast that with today, where the introduction of cheap, easy to apply switching ICs has made the coil choice abundant.

Build your own, once you choose the right wire for the antenna and dielectric for the center support, there's not much to screw up.