Stack Overflow (and all other sites in the SE network) are more strict because the goal of all these sites is to build up a collection of valuable knowledge for future visitors to read and learn from.
For that reason the quality standards on the sites are rather high and content or behavior that is deemed not to be a good fit for that goal is moderated fiercely by the members of the community once they earned the privilege to do so. So far the balance is that a few blocked users doesn't hamper the overall content quality, so the bans and throttles work.
The Stack Exchange team developed a wide range of techniques, data collection and matching strategies to either ban or throttle the admission of content that originates from certain IP-addresses or even IP-ranges. For low reputation accounts these throttles are a bit more tightened. Once an account gains more reputation the throttle is adapted as well until it is hardly noticeable.
Some of the throttling / banning is done at the edge of the physical computer network of Stack Exchange which lead to the blocking of traffic from certain IP addresses across the whole SE computer network. Other throttling is done at network account level and some at user level, which is per site.
Even under normal operation actions from users are rate limited as is explained in The complete rate-limiting guide.
So why on earth is Stack Overflow more strict than other sites? Because we care deeply about the content found here and that is best preserved to slow down those users and/or computer network that submits sub-par content, allowing them to spend more time reading and learning the ropes of contributing on the Q/A sites in the SE network, instead of letting them go and the community have to clean-up afterwards.