Why do people keep saying the Big Bang Theory is a fact, lol

"It's a fact, and doesn't require any 'belief'."?
Then why is it still called the Big Bang *Theory*..?

There are no 'facts' in science. There are only hypotheses supported by evidence, and hypothesis not supported by evidence. All evidence collected so far points towards the Big Bang hypothesis being true, and that is why scientists believe it.

The Big Bang is the model (description) of the universe that is best supported by all cosmological observations.

Since the universe is seen to be expanding today, theoretical cosmologists can make detailed predictions of the universe's denser and hotter past. The results of Big Bang predictions have been found to conform accurately and precisely to a variety of astronomical observations.

(The word, "theory" has several different definitions.)
.

"Theory" in spoken English has a different meaning from "theory" in science.

In science, there is a hierarchy for factual evidence and the steps of the scientific method. First of all, observations are made that show something new. Out of these observations, a hypothesis is drafted, which attempts to explain the observations. Then, experiments are performed and more observations are made to see if the hypothesis corresponds to the data obtained. The hypothesis is adapted as necessary, and tested again, adapted again, tested again etc. until the observations can be predicted and correspond to the hypothesis. At this point, the hypothesis will become a theory, which has been tested extensively and has proven itself numerous times.

It is very possible for a theory to be overturned by a single observation. If a Homo erectus fossil is found next to an equally old Homo sapiens fossil, for example, the theory of how humans evolved from Homo erectus will be invalid, and a new theory will have to be developed.