Some of it is just market forces: originally smart bulbs cost $60, but competition has brought the pricing down over the last 15 months or so.
Some of it is true costs: it is much more complicated to engineer something that gets actually wired into the mains rather than just screwing in like a bulb. There’s additional safety equipment required for fire protection, in part because you have to allow for the possibility of someone doing it wrong. 
And the actuator part of the switch also requires a bit more equipment and complexity. But not a whole lot more.
I do expect the cost of smart switches to drop over the next two years, but no guarantees and no idea how much. But if I had to guess I would guess that by the end of 2019 smart light switches will be in the $19-$22 range. But we will just have to wait-and-see.
By the way, the issue of having a wall switch that can work with a smart bulb without cutting the current to it can be solved in several different ways. 
However, they all require adding additional devices, which then increases costs and makes a Bulbs project cost much more like a Switch project. But a lot of community members, including myself, do use one of these methods. See the following FAQ (this is a clickable link)
But for now, I think we are still paying early adopter prices for the switches, even though we are actually many years into home automation. SmartSwitches cost $35 because the people buying them are willing to pay $35. As the numbers of those interested in the devices but unwilling to pay those prices increase, I do expect prices to drop.