some borders are also based on really, really old treaties in europe. A lot of borders in regards to provinces are based on old counties and baronies for example. Baarle-hertog/baarle-nassau being a famous example of the inconvenice it results in.

Prior to world war 2, this was also the case inside germany, considering many Lander still had lands all over the place (especially prussia) which made governance a hassle.