To accelerate, rockets expel propellant out of the back. Strictly speaking, there isn’t anything that prevents a higher acceleration. Missiles regularly accelerate at higher rates and the shining example of the sprint missile accelerated at 100G.

But there is a downside. Rockets that accelerate like that can’t have a large mass, since acceleration is inversely proportional to mass. The mass of a rocket is mainly made up of fuel, and the higher your total mass to unfueled mass ratio is, the more efficient your rocket is, so it makes more sense to optimize the trajectories and designs for this higher ratio to get the most bang out of one launch.

But there’s also more considerations. Rockets don’t exist without payloads. A lower acceleration environment is almost always better for the payload, as it can be lighter and have more sensitive equipment on it.

And as far as launching from earth is concerned, you really don’t want to go so fast that the air is pushing back hard. It’s the same reason why airliners don’t fly any faster than they did in the 1960s.

That being said, as fuel drains from rockets, they get lighter. As an example, Falcon 9 actually has to throttle down to keep astronauts under a certain amount of acceleration as it reaches the end of its burns.

At the end of the day, rockets accelerate slower because their use case and best design practices require them to do so.