Because they weigh a ton.

I did this to my giant defy3 when i got my CX bike frame, now it has a triple, i replaced the BB, rear derailleur, my shifters and probably will change my front derailleur too, it works but can't use the lowest chain ring..rear derailleur needs to be long cage and the front too..EBAY IS YOUR FRIEND..

I used to do this ride with a 39 -23 low. I recently did it for the first time in 20 years and was very happy to have a triple!
https://ridewithgps.com/routes/3377567

24-36-46 with a 21-11 eight speed. Low dollar way to get a durable,CHEAP, close ratio set up with a 175mm crankset.

I actually went the other way, to a double from a triple. Old bike had a triple (52/39/26), and a new bike had a double. Gearing on the new bike was 50/34 and 11/30 in the back. A tad too tall as it is quite hilly, on the verge of mountainous in some areas around here. My friend had been running a White Industries VBC crank for sometime that was a 48/32. Paired that with the 11/30 in the back and that was enough of a reduction for me. Sure I lost some of my top-end speed(and for sure I needed to gain some strength as 26/30 had been my previous low gear), but 32/30 is fine, and the 48 covers anything that wasn't 5%+. Not a cheap crank though. I will have to find a way to scrape up the money once my friend wants his crank back