It’s pretty easy to blast a hole straight through a thin tube brazing as well as TIG welding. I think it’s primarily a fashion/difference thing. THat said, there are a number of production frames which still use lugs, as well as a number of custom builders which TIG weld (Strong, Seven, IF, Goodrich, Crisp + basically anyone who does Ti, and a whole host of others).
Don’t lugs pre-determine the geometry? Or are the lugs custom made too?
There is a variation of +/- 1 degree within any given lug, and lugsets often have a couple of variants of the main tube junctions so as to allow quite a wide range. That said, yes it can be constraining in terms of geometry to design/build a frame with lugs, but not to the extent it won’t be custom. Yes, it possible to fabricate lugs, but to actually cast your own lugs which will be sufficiently strong is a very involved process for a one-off frame. Often the route taken is to adapt existing lugs, or to make something which looks like a lug by brazing together tubing then cutting. This is done purely for aesthetic reasons though.