Phosphoric acid detergents will pit your blade. If the knife is not a stainless steel, the wash and dry cycle will cause accelerated rusting. In wooden-handled knives with a rat tail tang construction, you can start destroying the handle from the inside out due to gaps in the construction allowing water seepage and degradation. In non-stainless knives, that same construction becomes the point where rust tends to build up.

Then you also have the action of the dishwasher water jets bouncing the knife around, dulling and destroying the edge.

Only the shittiest cheapest plastic-handled knives I own touch the dishwasher. Everything else gets cleaned and wiped by hand and put straight to the knife block or its respective scabbard.