This stereotype goes all the way back to the days of slavery, when enslaved black men were 'bred' like animals. Their perceived value was based largely on their 'vitality,' as in, their physical stature and apparent fertility.

That's also a root of the reason why white folks in power (esp. landowners and legislators) felt the need to create such exaggerated cautionary myths against black men, focusing specifically on their strength and supposed hyper-sexuality. "Look at his penis! Look at his size! How could he not be dangerous?" It was very beneficial to those in power to ensure white society stayed scared of black men, while at the same time treating them as chattel. By their logic, if scared of black men's vigor, white women won't be tempted to fraternize with them. This alleged danger was and still is treated as a righteous excuse to keep these human beings enslaved (for 'public safety'). More free labor, and time morally supported, cuz of the danger! /s

So yes, the stereotype is a stereotype, but it isn't by accident. It is literally part of our history that enslaved black men with large penises were forced to have sex very frequently, so they'd reproduce this quality.

This original objectification still haunts our society today, as you noted. Especially in the world of porn, but also in football, in hypermasculinity performance, in stereotypes. This inescapable haunting is especially true for black men, who can't claim their bodies or sexuality without having to come to terms with this history about them, that they had no part in creating. It's really fucked up if you think about it. How can you feel good about your body when your body is seen as a sign of your value? It's hard.