Because at that point in a piston [a constant power variable thrust machine--as
Bookworm explained] you have the lowest drag coincident greatest possible lift coincident the only thing affecting climb rate as such is the difference between POWER required and POWER
available, however as the power falls off with altitude while drag is always equal to thrust....
But, ROC will decrease so to maintain the highest possible ROC [the condition required of Vx] at less power [from critical altitude] an increase in the lift term and therefore angle of attack is required this action induces drag and requires greater thrust maintain that new AOA at a constant altitude,...i.e there's less actual drag at altitude, but more drag required because more lift is required, so more thrust is required,...READ CAREFULLY THIS!!!
And, since power is thrust X Vtas and increase in velocity is required to maintain the required power for max climb angle,...
so in pistons Vx continually increases until it converges with Vy at the absolute performance altitude, whereas a jet produces constant thrust and a constant the AOA is that which produces the greatest climb rate i.e the AOA for L/Dmax
Oh yeah listen to
Airfoilmod, he's cool and your procedure may kill you land straight ahead with only shallow turns to avoid obstacles,..as most flight handbooks advise
PA