Why are devices limited to 29 waypoints?

Just because it is, can't ever recall a reason being stated. Having said that I've been planning routes for Garmin devices for over 10 years, can't recall ever getting close to 29. If you ever do you could split the route into two.

I should be a little more clear.  If planning a long route for a group, you might need more than 29 "waypoints" to fully define the route.  This is to ensure all GPS devices calculate the same path.  Shaping points then become extremely important. Some other route planning software cannot create "shaping points", only waypoints.  Routes imported from Scenic, as a GPX file, for example, have this issue.  Routes created by Tyre can have shaping points but the route creator must remember this if exporting a GPX for use in a Garmin device.  Generally, group users of Garmin devices are not aware of these details and find it frustrating when their device randomly decides to split a route in two.  It's just weird that 1) Garmin devices have this limit and that 2) is an odd number like 29.

You can always import a 3rd party route into BaseCamp and alter it appropriately.

Regarding the 29, it usually boils down to memory.  There are always limits to how much data a device (without accessory cards) can handle.  Depending on how the software works, there may be reserved space for information that is stored for different objects (routes, tracks, waypoints) beyond the gpx files.  This space has to be on the device.  Backwards compatiblilty can result in a limit moving forward even though there is more memory available in newer devices.  Modern Garmin devices use shaping points to get around the limit of Waypoints in routes.  However, this is a Garmin extension to the gpx file format (i.e. it is not part of proposed format- but the format does provide for the use of extensions).

Tyre looks like it is TomTom specific.  Most sites providing files for hiking, biking or motorcycling download or include a Track.  It really is up to the individual to convert this Track to a Route (if required) that is compatible with a specific device and map.  A Route that works without a hitch on a device with a Garmin Map may run into issues on the same device using a third party map.

BaseCamp should have no issue importing a gpx file that uses only waypoints.  But it will be up to the user to convert that Route to something that meets the limitations of a specific device.