They may be small, but SFF cases are almost always incredibly well-made. Fewer materials are required to craft them, but more often than not, those materials are the highest quality.
In addition, as things get smaller, production requires greater precision and different techniques. You may have also noticed that SFF cases are actually something of a rarity.
Being that there simply aren’t as many cases for SFF builds as there are for other form factors, their scarcity, comes hand in hand with value. They’re like rare gems, while mid towers are mere pebbles.
One last reason you shouldn’t expect smaller to mean cheaper is that manufacturers need to keep up their profit margins. These little things are harder to produce and don’t sell nearly as well as your standard ATX mid tower case. Bumping the price up a bit ensures making SFF cases is worth the resources they put into making them.