Because when you use spacers they pull the axle back towards the cab and off centers it to the drivers side, the higher you go te worse it is
When you go more the 2.5" it leave your driveshaft on a harsh angle and will cause driveline vibrations
A2.5" spacer isnt 2.5" either
Say your a 19 yr old welder, you making atleast $12 an hour, you spen $40 in material and 4 hours cutting and welding your already over the $80 HBS charges for a 2.5 computer welded spacer that was designed by an engineer and yours isn't even powdercoated.
With 33" tires you shouldn't have height problems....