@shesgonebatshitagain
I do agree in theory but in practice it's not always as clear cut.
For instance, in my case, my DP's ex's family had all moved away so her only tie to the area was DP and his family. When they split she, understandably, wanted to be nearer to her own parents and support network. Her mum is also not very well so she always wanted to be closer to help her out more. My DP has his job and whole family in the original area so he wasn't going to follow his ex to where she moved.
I guess it's a difficult balance to ensure the kids spend enough time with both parents but also don't spend too much time travelling or missing out. In my DP's case, if the kids have birthday parties or sports events to attend, he goes up to them and stays at their home while their mum goes to her parents' for the weekend.
Obviously it's different scenario with OP's ex but there must be a reason he moved to where he did, especially as it seems he still wants to see his children just as much. Perhaps it's the only job he could get or the only one that paid enough to cover his costs or whatever.
So yeah I do agree, ideally parents wouldn't live 100+ miles apart but when that is the case, for whatever reason, things that make it better for the kids are a priority. If each parent does half the journey, they could meet somewhere where the collecting parent can get them some dinner as a break in the car journey. If one is doing the whole way it will be much more arduous for them.