Why is electric cord thicker than it used to be, and do they make
little roller switches for thick cord?

The first question is philosophical and the second is practical.

Lamp cords and fan cords etc. are thicker than they were years ago. I
don't think the copper is thicker but the vinyl is? Why did they do
that? The old cords were fine (and variious designes of
quick-attach plugs, receptacles, switches, etc. were suited for the
old thickness) .

(I haven't rewound my 78 year old fan yet, but I do like the $5 dollar
one I just bought. It fits on the windows sill and I want to put the
same roller switch on the cord that the other fan had, so I only have
to raise my arm to turn the fan off.)

But the cord is so thick, the only inline switches that will work are
the 2 or 3" long ones. going by what I've found so far

I've been to Home Depot and I've looked online. I've looked on Ace
and Lowe's web pages.