You probably didn't know it because it's not actually so, at least not in general. It's actually a restriction on the maximum velocity of a Bussard ramjet relative to the interstellar medium¹ because you have to accelerate the incoming material. You could go faster² by switching over to internal propellant reserves once you approach that maximum speed.

There _is_ a practical limit for total deltaV of about three times the exhaust velocity because that's where the increased propellant requirements to move your propellant tanks reaches the amount of propellant those tanks hold.

(You can 'cheat' this by using staging because dropping engines/tanks/etc. increases your mass ratio.)

1: If we were to assume that Bussard ramjets actually worked, which they don't. They would have so much drag that it would be impossible to get any net positive thrust from one.

2: Again, assuming they worked as anything but a brake.