And not that many use invis potions.

Detect invis Scrolls are expensive because of their short
duration.  (I used them with my last thief just so I'd improve
at scroll use over time, until I decided it was too expensive
and too short duration.) Black roots last ages and really take
the effort out of countering invis.

Depending on your class there is also free detect invis in the
form of talisman lying around.

Also, water generally stops invis working if you are under
water.