there are so many reasons for that

-there is sometimes not a single statical exe (that means all code inside) but overlays(DOS like DLLs) or serveral other ways of loading code at runtime (example for sound/gfx-drivers) - DOS allows technicaly nearly everything so everything is done in games :)

-many game loaders combine code/data parts of a game in memory - for keeping floppy releases smaller

-self modifying code, also hard to disassemble statically with Gidrah/IDA

-good old segment/offset 16bit realmode games - a complete different beast compare to 32bit linear DOS games (Ghidra isn't very good at this, IDA is much much better)

some examples:

the Stunts loader combines several (in itself non valid) files in memory to create a exe (the single files are packed and the result in exe in memory is also packed) - not that easy to static disassemble something like that

Alpha Waves also got an loader and self modifying code that is not easy to reverse statical

its good to have the best disassemblers available and the best (or better dedicated) debuggers around to keep your reversing project shorter then decades :)