Because:

1.  Some Jenkins users are less technical or not technical (QA testers) and may therefore be unaware of the consequences of pasting a string with extra space characters

2.  Jenkins prohibits changing params.* values from within the pipeline and

3.  Repeating the params.myStringParameter.trim() with each use of a parameter is tedious

The build-with-parameters-plugin should be enhanced to automatically trim string parameters before storing them.