Of course you could put up a fence. This isn’t a situation where the collars solve a previously unsolvable problem. But fences cost money, both the build and to maintain. Fences restrict movement for people (and wildlife), which is undesirable. You have to build gates for people to use, and if it is a busy area you need a lot of gates. People fail to close gates and cows get through them. If you need to move vehicles or equipment between the canal and the field, then you need some large gates as well.

The collars allow for restricting the movement of cows, while leaving the entire area open and unencumbered for everything else.