Because of several reasons
(1) poorly documented history as well as not having carbon dating until somewhat recently so the dating of the last ice age is off from what we originally believed which means that the ice has been melting for a longer period of time then originally thought.

(2) when the industrial revolution became a global entity there was no scientific knowledge available to say what could possibly happen once we fired up a few thousand non filtered coal burning furnaces.

(3) same thing with all modes of transportation.

(3) No body has really gone there but we’ve detonated a hundred plus nukes in past 70 years.

(4) space missions. This is still theoretical and because of our future intentions it’ll most likely never be investigated. But each and every time that we push through our atmosphere we have to be doing something to it especially when consider that we’re burning fuel which leaves an exhaust.

(5) lesser cooling volcanic activity. Several volcanologist have stated that there’s two types of volcanic emissions of greenhouse gases.
And depending upon the type it can cool our planet down or can actually amplify the results of global warming.

So basically it’s a combination of our own ignorance and nature. Well that and our population vs nature itself has become unbalanced so this planet is going to preserve itself with or without us.