There are a lot of other pieces of data that can be put into URL parameters besides
UTM data. For example, YouTube set a parameter "v" which is equal to some ID. Different IDs result in completely different pages. They also use other URL parameters (such as "t," which corresponds to a time in seconds where the video should start playing) to indicate other data. Some examples:
- youtube.com/watch?v=hEcjgJSqSRU
- youtube.com/watch?t=100&v=hEcjgJSqSRU
- youtube.com/watch?v=hEcjgJSqSRU
- youtube.com/watch?v=cvkBvzpbBPs
The "t" parameter doesn't affect the actual content, but the "v" parameter results in completely different pages being shown.
Since those types of parameters can be set by web developers to do literally anything they want them to, there are a lot of scenarios where ignoring them wouldn't work.
Which is one reason why the canonical tag is used. Presumably Majestic can understand that the "t" value in YouTube URLs isn't a different page because presumably YouTube set canonical tags. A site that doesn't have canonical tags set can't be understood easily by backlink checkers.