There's 2 ways to make 2666MHz ram. The first is to take new silicon chips, test it to see if it's stable at 2666MHz, put a sticker on it and add it to the batch that was demanded by the buyer. The second is to take silicon from a batch that failed a higher speed demand, like 3200MHz, and test it to see if it'll pass 2666MHz instead.
With the first batch, the top end is unknown, it might OC to 4000 for all anyone knows, it's never been tested for maximums. With the second batch, the top end is known, somewhat, as it failed a higher speed at least once. None but the factory have any clue what that might be, it might have been 2800MHz, so any OC applied to those sticks will be doomed to failure.
Heatsinks. Value ram does not have any. Simply meaning that an xmp (eXtreme Memory Profile) that will change a 1.2v ram to 1.35v ram, is going to create higher heat loads when used. Without the ability to dissipate that heat, the ram cooks, and there's no temp sensors on it. So value ram won't have an XMP profile. It'll use standard jadec tables and run the best speeds allowed by the processor, like 2133/2400 or 2666MHz.
You can try to overclock it manually, you can try to tighten timings, but with value ram runs a much higher than normal chance of failure, errors, damage if OC is implemented.
All due to excessive heat, excessive voltages, lousy binned silicon or even substandard silicon. It's one of situations where 'Yes you can', but conventional wisdom says 'You really shouldn't'.