Used phones have actual human usage history associated with their hardware identifiers. If you were to use emulated devices with spoofed identifiers, it would be easier for the large social media services to filter out the bot traffic.

Using old devices that were once used by real people leaves a digital paper trail that makes it harder to make algorithms to filter out bots.

It is actually incredibly hard to create a perfect emulated environment that cannot be detected by the software running within it, in this case the official social media app the bots have to use. The apps of the social media platforms cannot really be fooled by an emulated environment. There are too many ways for it to check whether it is running on hardware or in a virtual machine or container.

Case in point ask anyone who has ever tried to use a graphics driver with windows in a virtual machine.