I often hear sentences (mis)constructed in this way.

Problem:
There is a singular/plural mismatch.

Explanation:
The phrase “a lot of cats” is singular, even though “cats” is plural, because “a lot” is singular.

The “are” form of the verb “be” is plural.

So there is a mismatch in number between “are” (plural) and “a lot of cats” (singular).

Solutions:
“There is a lot of cats here.”
or
“There are lots of cats here.”