If it was up to the teams they could have just went with any game at any time against anyone they want and that would work for them, but since it's now all decided, they are facing teams that UEFA tells them and they play on the dates that UEFA tells them, which is limiting.
But it doesn't take the League of Nations for that to happen. This has always been the case, all matches of the national teams are carried out according to the FIFA calendar and continental associations. Even friendly matches are held on dates defined by FIFA, in this case what differs is the choice of opponent that is made by UEFA. It should be noted that all over the world national teams played games, friendly or not, on the same days as the Nations League games.