Because it can't auto replace the obsolete cars, so if in hundreds of obsolete cars, which way is the best to replace it?
Buy new vehicles, release them from depot - and then there are two ways:
1. Send vehicle into depot (and then sell)
2. In vehicle dialogue use option described Sell vehicle after full unloading.
The second way is a little slower, but better.
You can also withdraw all the vehicles on a line with one button in the line management window. (I think they stop loading new cargo/passengers, and once empty they are automatically sold.) Replacement vehicles must be built by hand, but as new vehicles often don't replace the old on a 1:1 ratio, figuring out the correct number of replacement requires human interaction anyway.
Most of the time, I do replacements vehicle for vehicle, though. For trains I mostly have to, since I only want to change the locomotive or the wagons. For buses, it is because buses in pak64 constantly increase in capacity, but at a point the newer buses are too big for some of the smaller lines, so I keep some obsolete buses in reserve. In all cases, replacing one vehicle at a time minimizes disruption of traffic/service.