Why don't towers do % true damage

Thats ridiculous.

A bit of % true damage on towers would be fine but not so squishies and tanks take the same damage.

Because tower diving would be impossible at that point. Diving for a kill is a mechanic they don't want completely removed. They just need to increase tower damage by a bit.

Nexus obelisk (the fountain turret) deals 1000 true damage per hit.

It would be balanced if it did %hp true, towers are worthless at the moment.

Maybe not 20% though.