I think PC game streaming with Steam Gamelink and Nvidia Shield handheld/tablet is not very popular because most PC gamers prefer keyboard and mouse. I think games also look better on a PC Gaming monitor with a fast refresh rate, and 4K resolution. Plus, many home network and internet connections are not fast enough for game streaming without video slowdown problems.
I agree they are a hassle compared to using a HDMI cable to plug in a PC to a TV if you want to use your TV for game streaming.
Plus, many gamers probably already have a TV and gaming PC in their room, so these game streaming devices are not needed if your PC and TV is in the same room.
Gaming laptops are also becoming more affordable where it maybe more worth it to save up $700 to $1000 for a new or used Gaming laptop for using to play games on the go, or to plug it into a TV, and not deal with using game streaming devices which can have slowdown problems from a slow network connection.