Because actual leftists reject the entire existence of the state, and therefore its institutions. We know that electoral politics are nothing but theatre that is corrupt by design and exists solely to provide the illusion of choice to placate the public, and manufacture consent for our own oppression and that of others. Even if we thought there was any sort of merit to it, we know that no one who threatens the system will ever be allowed to get to the top of said system to "change it from within", that's just another lie perpetuated to maintain the status quo (as long as people believe that elections are the only way to change things, and keep trying that despite all evidence pointing to the contrary, those in power get to keep it).

If you genuinely care and want to know more, you could start here:

https://theanarchistlibrary.org/library/the-anarchist-faq-editorial-collective-an-anarchist-faq-full (for some reason I can't link to specific sections of the document, it's a bit overwhelming, but looks like sections B.2, H1.5, H.5, and J.2 would be a good place to start, though it really is worth working your way through the entire thing)