Is there any reason to not upgrade from Sierra to Big Sur directly?

Not really, but do make a backup first in case you discover there was a good reason not to.

Back up your data in case something goes wrong --> Back up your Mac with Time Machine - https://support.apple.com/HT201250 or use a cloning tool.

Also check if your old apps will still work with with the OS you want to install - https://roaringapps.com/apps This is particularly critical if you are upgrading to Catalina or newer, as older 32 bit apps will no longer work. For those, try this 32 bit application scanner application --> https://www.stclairsoft.com/Go64/