Easy because the small size make the leads heat quickly and draw in the solder. The solder is also drawn to the pads. This can help center and align the components. As the solder is pulled to the pads at either end, the part is slight shifted to balance the forces.
The hard part is the parts can be tiny and move around with the slightest touch. This can make it a challenge to position the parts and annoying because they move around just touching them with a soldering iron.
After learning to make my own aluminum stencils I decided that SMD parts, even with fine pitch pins like on a 32-pin TQFP, are really easy to use.
The secret is in the solder paste and how it works.
Solder paste is thick. When it is cold, it can hold its shape. When you first apply the paste it will look nice and each pad will have an individual patch. Then as it warms up the paste will flow. A part with many pins will become a grey blob.
It does not matter if the solder paste is a blob instead of individual patches. Magically as it is heated, the paste for individual pins will form distinct patterns to match the pads and pins.
I learned that it was more important to have the right amount of paste, and less important about how nicely it was applied. Too much paste and it would create solder bridges for me to fix. Too little paste, and either a bad connection or the whole part could be drawn to the opposite pad.