Pre-rooted phones are a niche market. As a practical matter, I think you should settle for one that is easy to root.
One of the few phones to come rooted was Fairphone 1, but it isn't manufactured anymore and doesn't have Android updates after Lollipop. Fairphone 2 comes with a non-rooted OS by default, and has an official rooted OS image, but the official rooted image is not being updated. However, it is supported by LineageOS.
Fairphone is a fair trade phone: they strive to ensure that the whole manufacturing chain from mining to assembling respects decent labor standards (working conditions, pay, etc.). It's markedly more expensive (and also heavier at 168g) than other phones with similar performance. Buying a Fairphone is a social gesture, not a purely consumerist purchase. Fairphone documents the cost breakdown (first edition). On the upside, if some part breaks, you can order a spare: you won't get stuck with a battery molded in plastic!
Fairphone's attitude towards openness is somewhat ambiguous. They prioritize fair trade over software openness, so some components use proprietary drivers. However, they are sufficiently committed to openness to ensure that you will always get an unlocked bootloader.
I have a Fairphone 2 and I'm happy with it now. However, until someone managed to make a LineageOS build, the choice was between an obsolete rooted OS from Fairphone and the official non-rooted, up-to-date OS.
If the important thing is to have a phone that you're sure you can root, I feel the best advice is to get a Google flagship phone. These are designed to be development phones and so come with an unlocked bootloader, they have official OS updates longer than just about any other phone, and they tend to have good community support (through CyanogenMod and now LineageOS).