Vent liners aren't installed in a barrel for ease of cleaning, they are a replacement for enlarged vent holes.
And yes, drilled vent holes were the norm for centuries,.... and used until the drilled vent hole got burnt out, then they were repaired with a vent liner, sometimes even made of gold.
Modern manufactures install vent liners in new muzzleloaders simply to save someone else the trouble of "drilling and tapping" for a vent liner hundreds/thousands of shots later.
Nipples on "bp revolvers and caplock rifles" don't screw into the gun's barrel, they screw into a much cheaper (and easier to replace) component of their parent host,..... UNLESS we're talking about inlines, under hammers, and mule ears, and those never became very popular.