When I was struggling to figure out why so many of my devices on my gigabit network were only getting 100 megabits, I learned that a lot more of my devices were on 10/100 ethernet than I expected. Including some recent purchases (TV's especially).
It took me a while because I assumed gigabit had been kind for far longer than it actually was in practice.
TV stuff doesn't tend to need more than 100Mbps, unless you're playing blu-ray 4k discs which have higher peak rates. Saving a buck or two on the SoC adds up for the manufacturer.
Yes - I think in cases where you have a good wireless setup at home it's actually preferable to not hardwire the TV. It's easy to get better performance over wifi when the port is only 100mbps.
Gigabit is more expensive, consumes more power, and generates more heat. If a device doesn't need the speed, 100M is a better choice.
Even the most high bitrate streaming services have peak bitrates under 80Mbps, with averages being much lower than that.