"You are too late!"
了 is actually a difficult word to learn (and for me to explain).
In this particular sentence, the 太…了pattern is used to express a strong emotion (e.g. anger, disappointment) and thus it is not suitable to omit 了 here.
However, when 太 is used purely in the meaning of "Too" (without a strong modal expression) 了 is not always necessary. e.g. 对不起，因为你来得太晚，已经卖完了。/ Sorry, because you came too late, it has already been sold out.
It is a particle that marks a change (e.g. perfection/completion) of the condition, and further be used for exclamation here.
No real reason I feel. 了 is a very confusing character and sometimes is just there to round off the sentence's sound
In this case, 了 is not a completed action marker. In this case it is simply a grammatical requirement. This is the structure 太…了 which means "too…". You cannot omit 了 here.