Without knowing which videos you’re watching in particular, I would guess that you’re under estimating the braking ability of some drivers.
Braking technique is definitely different from what is used in a car. A kart will generally perform better with less trail braking under normal circumstances, and a driver may trail brake a bit more when overtaking.
To TJ’s point, any top driver is going to be braking at the absolute limit for every corner of every lap. There isn’t a magic “brake later” button that drivers can deploy in order to suddenly gain an advantage. Instead, overtakes are often accomplished by capitalizing on mistakes, and/or using momentum from a previous corner to initiate an overtake down a straightaway into a braking zone. When everyone is braking at the limit, overtaking can often be easier said than done.
Here’s a video of a top-level driver, Matt Jaskol, in a stacked KZ field at the Supernats. You’ll notice that although there are many braking zones, not every one is necessarily an equal passing opportunity. Overtaking is a skill that takes quite some time to develop, and you’ll see even the best drivers make mistakes from time to time when passing.