A small anecdotal example of why this complicates fighting the pandemic: during the first wave, eastern Germany got off relatively easy, with few cases. Czechia was even better, because they locked their borders and hardly had any cases initially. Then, during the summer/early autumn, Germany had the usual rules with masks, distancing, no large gatherings etc., while Czechia had much laxer/less-enforced rules. So guys from Germany would throw their bachelor party in Czechia, get infected and then infect everyone at their wedding a few days later. Which led not only to Czechia having a catastrophic number of cases in October, but also to eastern Germany (particularly Saxony) being hardest hit in the second wave.