You don't have to, you want to. Nobody is making you do it. Just so we have semantics right, and we can tell the widows something.
It will cost you $20,000 to fix your tele, if you bend it. I've set up half a dozen that have been laid over. I was on a jobsite where a 9k picked up a 12,000lb manlift off a trailer, foreman told the operator "it will be fine, we've done it before". It picked the manlift fine, then dumped it off the forks on the way to the ground, after the trailer pulled out. That was a $95,000 mistake, they scrapped the manlift afterwards.
Just for a example, most rubber tired lifting equipment is built with a 15-25% safety margin for capacities. Most skid loaders show a rated capacity at 50%, to deal with side slope factors. Cranes are rated on tires at 75% of capacity. Telehandler manufactures don't show their %, I think just to keep people from doing the math and pushing the limits, trying to limit their liability. You're looking at around 140% of capacity pick. Which makes you a idiot if you do it. It might work, it might not, but equipment is pretty expensive and people are priceless.
Do you want to buy the truck driver a new trailer, if the forklift comes off the ground and it and the load, smashes back down into the rear of the trailer? (I've seen it happen) Do you want to make the phone call to the widow of the guy, that gets hit by a dunnage block that gets shot out from under it, when it falls?
"Oh we were only making a 140% of capacity pick, we were going to stop if something looked wrong." It will go wrong before you can do anything about it, if it goes wrong.
All that said, if I had a 10k telehandler with outriggers I'd make a 11,000lbs pick on level ground, outriggers down, with no boom extended. Which would be 10% over capacity, but that's my comfort level, I'm not about to tell you where you should be.
I will say - 140% is stupid territory, and with what insurance and workman's comp costs, why do that? Especially when its so easy to go rent a bigger telehandler that will do it, or if you have other equipment there to take part of the load.