How do you know that they get by perfectly well? Facts and figures please.
It's a fair question and I don't have any figures (knowing how many people successfully fare evade is hard to calculate by its very nature in any case).
The key issue IMHO is whether it's worth operating stations/station platforms as a sophisticated and expensive electronic fortress, which also needs to be manned, in an attempt to reduce fare evasion to as close to zero as possible.
Unfortunately even this system is not watertight against fare evaders, e.g. adults buying child tickets, buying tickets/season tickets short of stop in the knowledge that the destination in one direction is unbarriered, "rushing" through the barrier behind someone with a ticket, finding other ways out of the station, or the barriers simply having to be left open at certain times.
On top of that are issues of crowding at gates particularly on arriving trains and issues with access to platforms for those who want to help friends/family with luggage or simply see them onto the train.
The European, and particularly "German" system, where even U-bahn and S-bahn are unbarriered and bus drivers also make no checks, relies on ticket inspectors on inter-city trains and plain clothes random checks on local trains, trams and buses with spot fines. One of the key strengths of the checks is not only the spot fine but the potential public embarrassment to a fare evader who hasn't got the gall simply to front this out.
I guess they take the view that although some will almost certainly try to abuse this, resources are better used elsewhere then going to the huge trouble and expense of completely re-casting the system to a barrier one. And they may well indeed also consider a barrier system to be financially worthless and generally less desirable.