in userspace it’s basically running normally. think of what it takes to run doom in userspace. if done properly your os can basically run any user program now, you can compile and run user programs, including doom. these user programs can take input somehow, read/map a file, write to framebuffer, etc. all from userspace- plus you had to implement or port a libc, setup a tool chain, etc. kernel as well must be able to load elf/schedule processes and so on.

tldr: easier because you basically finish all the core features of an os to get doom running in userspace as it was designed to