Maybe it's gotten better since they first dropped, but this article alludes to the storage limitations. I ran into the problem with mine. I remember I had to delete apps and clear app data to get enough space to install an update.
The article also speculates the lack of storage may be why they Android version hasn't been updated on them since release.
Edit: Here's another link describing the problem around the time it launched. https://www.androidcentral.com/chromecast-google-tv-update-extra-storage-space-management
Edit #2: They may have received an Android update the first year after launch, but not since? Sorry but I've gotten a little fuzzy on the details.
Ah I feel you. Now that you mention it, I may have had to cast something when I was using mine because it had a phone app but not a Google TV app. Syfy maybe? It's been too long.