Speculation only, but I'm guessing it's because, depending on the circumstances of the emergency, it may be difficult to guess exactly where the aircraft will touchdown and then come to a stop, hence the strategic and dynamic positioning of the fire crew vehicles.
During a DL MD
-88 smoke-in-the-cabin emergency descent/landing at JAN (Jackson, MS
), on which I was a pax, the equipment was pretty much clustered around halfway down the rwy and they were therefore able to "ambush" my MD
-88 quickly with foam canons aimed at the rear of the aircraft (the air conditioning system was the suspected cause of the smoke which had quickly dissipated during the emergency descent once the crew disabled one of the air conditioning packs). No foam was required and after a few minutes the aircraft taxiied to the gate under its own power.
I'm taking an educated guess here, but I'm betting the JAN fire crews had been advised by the cockpit crew that:
1. The engines were both functional
2. There were no known hydraulic system issues (thus, a normal landing followed by normal braking were expected)
...and it was dry so normal braking friction with the rwy was expected as well.
On the other hand, my father experienced a #2 engine seizure after takeoff from DTW
back in the late 80s. The DTW
fire vehicles did a "chase" as you put it with a few vehicles "chasing" the aircraft on parallel taxiways on either side of his 752. I would imagine this might've been because of the unpredictability involved with an engine-out landing and assymetric thrust reverser rollout.