That's kind of amazing unless you just never play consistently on any single amp that much. I've had tubes fail prematurely, tubes expire, and tubes get blown by amp failures (I'm looking at you, Peavey). Even so, all my amps have relatively fresh power tubes and none of the amps are dodgy so I'm not concerned either...for NOW...
I guess maybe I've just been really lucky.
Actually I re-counted and it has been 3 failures. My first was my JCM 900 4500 that I wish I still had. I had a Mullard RI EL34 do a full on melt down and take out several fuses. Replaced with another set of Mullards. (great tubes by the way)
My main amp is a Rivera Knucklehead II. It had JJ E34Ls, and I started getting some unusual noises, and then one of the tubes started redplating. I also replaced those with a quad of Mullards.
Finally was my Butcher (my second one). I actually bought it around ten years ago and was amazed to see that it still had the original tubes which was surprising because the thing looked pretty road worn when I got it. I played it for about a year before it started making some strange noises and one of the tubes took a shit. I replaced those original 6L6s with a set of Tung Sols.
The 900 was my only amp for a number of years, so it saw excessive use at pretty good volumes. The Rivera became my main amp after that and probably gets played about an hour a day on average for the last ten years, and aside from the odd occasion it's been the amp I use when playing out. If I am remembering correctly, I replaced the tubes about 6 years ago.
Aside from that I've had a few preamp tubes go bad.
So yeah, I guess the ones that have failed were in amps that had seen a lot of use. Makes sense. The whole idea of replacing tubes every year though, I just can't get behind that. Seems like a pointless waste of money to me.