Okay. I want you to take that feeling--of having to invent the thing from whole cloth--and now apply it to every single time you use Custom Lineage to create a race/species that "already exists."The flavor is already in the Monster Manual, or Volo's, or in your favorite mythology book, or your favorite novel... whatever inspired you to play that particular character in the first place. It's as difficult as you want it to be, really.
Or you can create it yourself from whole cloth and pure imagination, as you describe. Which would be the most challenging part for me, because it requires me to first decide why I need a different species in the first place. But I've seen it done... my players love doing it that way.
Because, properly speaking, if we did things the way you're saying we should, then mechanically strictly no races/species "already exist." Zip, zero, nada. There is nothing written about "elf" or whatever, except your impressions of the literary canon of fiction. There is only the sterile Custom Lineage Creation Rules, and your perceptions of things. You are, each and every time, reinventing the satyr or the orc or the dwarf (or whatever), from the ground up. That's how your rather flippant just-use-Custom-Lineage-4head response comes across: literally an instruction to completely reinvent each and every race/species, each and every time you make a new character, with the one and only consistent throughline being "what you feel like playing." Which, as I think you would agree, is...not exactly much of a throughline.
I am sympathetic to both positions, just to be clear. I think it would be an exceptionally bad idea to trash all races/species and just have "here's Custom Lineage, have fun!" Yet, conversely, I absolutely recognize the power (in a design sense, not a "become overpowered" sense), flexibility, and utility of the Custom Lineage rules. They are extremely good at what they do, and what they do is enabling players to play what excites them--and that's a thing I support most strongly.
Trashing all writeups and just having fluff descriptions and "here's Custom Lineage, have at it!" would be very bad. It would suck a lot of the flavor, and the joy, out of D&D. At the same time, Custom Lineage is extremely useful, and should probably be a feature of future PHBs (or "PHB1," if there's more than one) because of that.