Why does nobody seem to design toilet seat fittings that stay put?

I have fitted 2 new different toilets in my house recently and the seats on both of them just slide around on their fittings. It doesn't matter how tight I make them.

Any plumbers have any tips for me?

Cheers

Mook

Ours stay still, I think it's because of the extra rubber bits though. From top to bottom there is;

Toilet seat hinge
Plastic cap
Rubber washer
toilet
Cone shaped rubber bung
Metal washer
wing nut

I think it's the cone shapes that do the best job, there's an example in this Ebay item;

TOILET SEAT TOP FIX GROMMET NON SLIP FIX REPLACEMENT HINGES BAR DUCK CHROME GOLD

Make sure your cheeks are well spread before you sit down, that way there's not as much squirming about to do...

^ fair point.

but I think the issue is that most seats are supplied with hard plastic washers, which have no friction/grip. You want some thin rubber washers in there....

We had this problem, in the first 5 years or so after moving into a new build we must have been changing ourbloo seats every 6 months, it was so bad I was keeping a couple as spares.
In the end the solution was simple and cheap we bought basic loo seats from IKEA. The design of the mounting has a solid mounting block which bolts across both holes in the porcelain and the hinge is separately mounted on this.
Other manufacturers do similar mountings.