Not every door frame is at the same level as the adjacent outside or inside floor. Once you get higher than 1/4" or so it starts to get a little tough to get a wheel to move over it, especially if it's a wheeled walker or user-powered wheelchair. In a powered scooter there can be an unexpected sudden stop or at least a significant bump when your loved one decides to use speed and mass to conquer what looks like a small vertical rise.
We have simple ramps made of recycled rubber or aluminum that can accommodate thresholds of up to 2½" and 5½". Since these ramps don't have vertical side flanges, doors will swing right over them. The rubber ramps do not extend over the threshold, making them ideal for sliding doors, and they can also be cut and modified to accommodate unusual conditions.
These are suitable for both indoor and outdoor use.