One simple reason is physical interaction.
lactose is only composed of two sugars, while cellulose is giant network of interlocked sugars. Cellulose keeps the sugars tightly packed in three dimensions. even getting to the bonding site is difficult.
human and many other lactases are a huge molecules that would never be able to fit on to a cellulose chain, while cellulase of often a tiny molecule as enzymes go and the active site is not buried in the center. Compare the two enzymes.
two different cellulase enzymes
Note the big difference in size. Also look at cellulase in action.
Notice it has to peel up the cellulose strip by strip, while lactose is just floating free in solution. This even limits the speed of cellulose breakdown. which means organisms that breakdown cellulose have to spend a log time doing it, while humans have a relatively short and fast digestive timeframe, so even if we could we would not get much from it.
These functional limits also makes a cellulase enzyme harder to evolve, since it can't be too large or in organisms with fast digestive processes. We often forget the physical necessities of enzyme function. there are plenty of other reasons enzymes do not function on multiple substrates, but this is why is basically impossible to for evolution to change lactase into a cellulose cleaving enzyme.