For the negative progressive of する, bunpro says its してない. Why is is not してないです? If the verbs just act as る verbs after getting conjugated to the てform why doesn’t it follow negative る verb rules? It also does not accept してません.
してないです and していません are both acceptable as Polite forms. For the most part, politeness markers (です and ます) are only used on the end of a complete sentence, so depending on the review you did, only the Plain form was accepted, I presume. It may be for other reasons as well, for example a very casual sentence ending in a polite way is very odd.
I’ll also add that it probably doesn’t accept してません because it’s a polite ending with a casual contraction.
In casual speech the い can drop
The most literal answer is です is an auxiliary verb that fits best at the end of sentences. Especially after nouns and adjectives. です after a verb (even in negative) is not the most common.
ます is also an auxiliary verb, and specifically sits on the end of verbs. So it will be more common when verbs are involved.
These are just linguistic habits, and neither is right or wrong beyond what is considered ‘normal’.
In direct answer to your question.
していない is casual
していません is polite
していないです is a polite form where the です is not required in reality.
Thank you everyone for your responses!