Can a PhD student accused of plagiarism revise/resubmit the thesis?
I think it's relevant what kind of plagiarism this is. 

If it's serious plagiarism, such as quoting large blocks of text without proper attribution, then that can be a serious academic offense. Asking why you can't just change it and re-submit is like asking why you can't just re-take an exam if you're caught cheating or why you can't just give the item back and forget everything if you're caught stealing.

Depending on your institutions procedures you might be allowed to resubmit if it's found that the plagiarism was minor, accidental or not actually plagiarism but I suspect you're missing the point. 

Plagiarism in a submitted thesis isn't like a simple technical error to be corrected, it's potential wrong-doing or dishonesty to be investigated.