Some of the posts boggle my mind in this thread, lol.
The soul gems are plentiful in Skyrim because the natural source of them is in Skyrim (well, one of them). If you've done the main quest, you've been there.
Welkynd and Varla stones are essentially Ayleid batteries. They're a way of transporting energy/magika. The Varla stones used to power Ayleid cities or at least aid in that function (IIRC there were larger stones again that were the primary power source).
The Dwemer use soul gems in a similar manner, they're used to power their automatons. Though how they do this is still unknown.
The Falmer, well, there is no evidence the Falmer ever had any complex or developed society. For all we know they were savages similar to the Forsworn. Perhaps even more so.
Yeppers. no real need for varla or welkynd equivalents. soul gems can be mined and soulgems are plentiful loot from dwemer automatons.
That and theres the perks that can make soul trapping and enchant recharging easier.
I actually have never soul trapped anything in the game after 200 hrs and yet i have an abundance of filled soulgems.