That's because heavy plastic liners are almost impossible to destroy. They'll take almost any kind of abuse and still look good and keep going. Here are three problems to watch for though. If you're going to be throwing something into the bed, then the plastic liners are likely the way to go. That's to prevent dents which are the way a truck bed really gets messed up.
Here are some problem areas to cosider however...
The plastic bed liners often trap water under the lining.
Over time that's a real problem. Standing water under the liner can be a perfect place for rust to start. You may not notice until it's too late. The rust is out of sight and can destroy a truck bed.
There's another potential damage spot.
Drop in liners often move around.
That may be simply from wind blowing under the liner or just from vibration. In either case, enough moving of the liner will wear paint away. That's bad enough, but that's another place for rust to start also.
More rust, or at least damaged paint, is what you get if the plastic moves around.
Many liners are noisy at highway speeds.
It's just the wind blowing under or around the liner. It's annoying if it starts and often there's little that can be done to stop it. This one isn't as harmful as the first two, just mostly aggravating... What's more, it's really a symptom of the moving liner problem.
Drop in versions, like the Duraliner and Pendaliner, are so tough, it's a shame they have these problems.
Drop in bed liners by DualLiner are made in separate pieces for the bottom, sides and tailgate. The bottom is a rubber truck bed mat made so it doesn't hold water. The other pieces are all tough plastic.
All the pieces then snap together to form the completed liner. That way you get a perfect fit and a great look, but the price is still low. Plus it's easy to ship and easy to install which is unusual for drop in bed liners. It's hard to get a plastic liner as big as a pickup bed shipped to your house if it's all in one piece.