Physics is a b***h, isn't it?
That's all, really. Even if you make the thing out of lithium-aluminium alloy, if the screen is a certain size and it fits in certain thickness and weight limits, there will be a point at which the thing is going to bend. There is no way round it.
If you really must put a phone in your trouser pocket, buy a small one. If you must have a big one, don't. But please do not ask for a redesign of the universe with (for instance) a larger value of the electric charge on the electron and the proton simply so you can have a big but not bendy phone, because neither Apple nor Samsung are spawning new universes to order. Yet.