Why am I prompted for a CAPTCHA when posting even though I didn't trip any of the rate limits for CAPTCHAs documented in The Complete Rate-Limiting Guide?
According to the comments, you are tethering your phone.
It is likely that your phone's data provider is doing carrier-grade NAT, which makes you share your IP address with a lot of other users at once, and makes you look less like a "real person." There is (probably) nothing you can do about this other than changing to a different carrier or using a different internet connection.
However, this answer suggests that CAPTCHAs can also be triggered by trying to do things too often or too quickly, as a form of rate-limiting. If that's the case for you, then you may be able to improve the situation by slowing down.