system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't we just keep accelerating in space?

I read recently that the space shuttle's top speed is approximately 8km/sec. However, since space is a vacuum and there is no air friction or anything like that to slow it down why wouldn't continually adding thrust lead to continual acceleration. What is it that causes the shuttle to top out at the current top speed?

EDIT: Thank you all very much for your brilliant answers. I'm having a fascinating time reading all the responses and the expansions and tangent discussions that have come from them.
gpt
Continually adding thrust requires continuously ejecting fuel, and only so much fuel can be taken in a given rocket. To take more fuel requires a bigger rocket which requires more fuel to accelerate, etc.