One of the biggest things Ttar has going for it is ease of access. You'll notice how most of the better options you mention are legendary and/or unobtainable:
-Shadow Claw Gengar is legacy and unobtainable
-Despite Niantic expanding the EX eligible pool, a lot of people are still without a Mewtwo
-The dogs and birds are out of raid rotation, and people who get them from research will need even more candies to power up than if they were obtained through raids.
Meanwhile Ttar has one of the longest raid campaigns going for it, bested only by Machamp (I think?), who turns out to be the best counter to it and even easier to obtain.
You'll notice those guides too also mention supplementing Ttar with higher DPS options for that very reason. Solo attempts should always start off with high-DPS and glass options and end with your bulky counters to get the job done. Ttar happens to have incredible DPS for being a tank.