The only logical reason why is down to your preference settings for BaseCamp on your home computer.
When you say “ I plan a route and it still redirects me to its own chosen route”, what do you mean exactly?
It can be a fiddle. As Wapping says, it’s down to your settings for the mode of travel, such as car, motorcycle, etc.
I’ve defaulted now to:
Motorcycle
Route type fast
No avoidances checked.
I then create a start and end waypoint, and then create a route from those. I then add a few way points, carefully to make sure they are on the road I want going in the right way and recalculate. Adding way points means clicking the map where you want to add them, naming for convenience maybe with a number so you can see which order to put them in, then opening the route dialogue and adding them in, in the right order.
If it’s still not what I want, I then drag the route to wherever I want to go.
Finally, I go back into the route and make most of the waypoints “unannounced” which turns them to shaping points.
I then create a track from the route and send both to the Garmin so I can see if I’ve gone of route anywhere.
Also, if I do want to stick firmly to the route, I switch off autorecalculate on the device, and make sure the device has the same maps and settings as Basecamp.
But I’m particular about my routes, and I want to go where I want to go. I have to do the same or similar using MRA or any of the other route planners as they rarely go where I want them to either.