Because they don't swim in pairs. Are there two of those fish? Nahhh.
Trouterspace.
A FSSSSSHHHH
If you know that one, try this one: Why do seagulls fly to the dump
Because everyone who runs, swims, or jump really well is already across the border.
A pair of legs.
Two-Baaas.
You can tuna piano, but you can't piano tuna. ... What about the pot of glue I knew you'd get stuck.
They keep getting caught in the net!