because in most situations wziąć/brać mean take.
Only some specific expressions have get=wziąć/brać.
Cambridge has only brać, but brać and wziąć are different aspects but mean the same
because that's how we say it. all the ęł verb endings sound like that, all ął verb endings sound "oł". (this is a source of many spelling mistakes)
Why is "I've taken my umbrella" not accepted? I'm far more likely to say this, as I own only one umbrella and am not going to take any random umbrella belonging to someone else.