As much as I’m inclined to agree with @MaggiWuerze@feddit.de, the real reason is typically that all new developments are required to include sidewalks, but existing ones aren’t required to retrofit. So you get a patchwork of sidewalks installed over time as things get torn down and rebuilt.

The “annoying and pedestrian hostile” part is municipalities’ unwillingness to infill sidewalks in front of old developments at taxpayer expense.