That is because a lot of RNAV SIDs and STARs have special leg types that can’t be replicated with manual waypoint entry. AIRNC 424 specifies 23 different leg types such as course to altitude, heading to intercept, radius to fix, DME arcs, etc. Of these 23 legs, only fix to fix legs can be easily replicated by manual entry.
If you find a STAR or SID that only has fix to fix routing, there’s no technical reason why you can’t fly it. You can perform the others with some creative flying, but it technically won’t be as accurate as a fully AIRNC 424 capable navigation system. Hence why the regulations require you to select all procedures from the database. Better safe than sorry.