You’re comparing apples and oranges. Or more accurately, you’re comparing standardized home automation protocols (Z wave, zigbee, HomeKit) with communication protocols that have no standardized home automation commands at all (WiFi and Bluetooth).
In a standardized protocol, there’s a predefined message that gets sent to turn a switch on, or set a dimmer to level 50%, or collect a sensor alert.
Because both the content and the format of the message are defined by the protocol, any manufacturer or developer can write a program to send that message, and the receiving device, providing it follows the standardized protocol, will know what to do with the message and act accordingly.
Without this kind of standard, each manufacturer is free to set up their own message format for their own device. So one light switch might expect the message “1“ to turn on while another light switch might expect the message “on“ and a third light switch might expect the message “tangerine armadillo.“
There’s no way to know what message the switch expects unless the manufacturer publishes that information. This is typically called a “open API.” Some manufacturers have one, such as Philips Hue. Most do not.
There are lots of reasons why a manufacturer might choose not to have an open API. Security is one reason. But to be honest, it’s more commonly because the manufacturer just doesn’t want to deal with customer support requests from people trying to write their own interfaces. Or using an interface that somebody else wrote. 
So… Cheap Wi-Fi devices don’t usually have an open API or an Ifttt channel or a HomeKit integration or a smartthings integration. And there isn’t anything that a smartthings developer can do to interface with a device that is not using one of the standardized home automation protocols that isn’t probably a whole lot more trouble than it’s worth if it’s even possible to begin with. It’s easier to just move on to a device which has more interface options. 