Two reasons come to mind immediately for the longer takeoff rolls...
1. Most takeoffs are made with flaps 1.
2. Thrust selection is made to accomadate something close to balanced field conditions. If we can safely make that takeoff with 22k vs 24k, we take advantage of MIA longer runways use less thrust and save wear and tear on the engines.
The 757/767 generally has Max thrust or standard thrust (slightly less). In either case, both these aircraft are generally takeoff with way too much power to get the job done. The 737NG "dial a thrust" system permits thoughtful selection of just the right amount to thrust to get the job done.
These I believe are the reasons for your correct observation that 737NG takeoff rolls are, in many cases, much longer than 757/767. However, at any point in the takeoff roll, the 737NG can select the full 26k thrust at any time if needed.