Fat frames are made with low density aluminium. More metal adds very little extra weight. You need fat frames for stiffness to eliminate flex which is bad for aluminum.
The manufacturers have done their math about volume production costs and aluminiumium is cheaper to fabricate than steel at high volumes.
Modern forks are made from carbon fibre and are moulded; you can make them any shape you want. As a designer, you can set up the correct trail and join the bits however you like. Straight blades use less material and are stronger and stiffer.
Old steel forks were manufactured by bending to achieve the trail. Late model steel race forks abandoned the curve and used straight blades welded at the correct angle but still with the same trail or offset.

You can still get old-school steel bikes from Soma, Gunnar, Rivendell, maybe Jamis.

/<Troll fed>