Because they reflect the intent of designers to lower the cost of transports. Eldar and Tau transports, which haven't been adjusted since 4th ed (when skimmers also had additional benefits that have since been stripped by 5th ed rules) do not. Sadly if 6th nerfs transports, or raises their costs again, Tau and Eldar will have to suffer while those 5e books get their prices raised again.

Likely though (read: hopefully) if transports get any nerfing the points cost will remain low to mitigate the down side. And then the additional hope that Tau and Eldar vehicles get adjusted soon.