It's a good question - unfortunately it's difficult to just repeat them, even if you didn't want to put any effort into improving them (I find a lot of teachers, myself included, are perfectionists and you also learn with experience what works or didn't, and see ways it could be better).
Different schools have different curriculums and methodologies, so moving schools last year to one with no shared resources meant I had to spend a huge amount of overtime creating PowerPoint lessons from scratch. The expectations were so different, it was less time consuming than adapting my old resources, for the most part.
Then curriculums change (the new GCSE curriculum in my subject has affected almost all year groups), and school expectations change, particularly if you're in a school that keeps introducing new CPD initiatives - or just has a particular focus on what they want to see in lessons that term/academic year, which is most schools. Learning walks add pressure to be seen to be ticking certain boxes. My department's subscriptions to different online textbooks and resources has changed each year, which affects lessons that relied on access to these (eg audio exercises)
Add to this, classes are different. What worked for a studious and able group last year is not going to work for a very mixed ability class with students with low motivation/interest. That's not even taking into account individual needs of students with SEND, low-intermediate EAL etc.
Having resources for the same curriculum from that school and low turnover in the department helps but you're never just 'done'. Though you learn to adapt on the spot more where needed.