Whilst plastic (polyamide) versions of the sump are cheaper to make, they do have other positive benefits too:
They are 50-60% lighter than metal equivalents. This allows improvements in handling, and as there is less mass this improves power to weight ratios (performance) and mass (thus improving emissions/fuel economy).
They will be quieter (2dB as a rule of thumb, which is quite a lot on a log scale), as it’s easier to design out areas that vibrate (like a drum skin) at the natural frequencies of the engine.
In terms of the vans with plastic underbody protection, the sumps have to be able to withstand impact testing to the same loads as the metal sump - why do you need a tray, if the sump can be designed to have addition strength to the equivalent of sump and trays?
By not having trays this makes it easier to maintain your car, and thus
servicing costs should be cheaper, but due to inflation remain about the same price.