Why Calc doesn’t have any increase/decrease font size keyboard shortcut?
It did exist in writer and Impress, but nothing in Calc.
Anyone know why the developers didn’t put that very useful function available for calc?
Because those keys are traditionally assigned to Mark Precedents and Mark Dependents (that is what people migrating from Excel expect).
In any case you can assign the same keys for Reduce Font and Increase Font (both functions are in the Category column under Format) following the steps in this topic
In Excel there is no keyboard shortcut, but there is menu to access increase/decrease font size.
Trying to modify LO Writer keyboard shortcut for those task, and when I look from Calc, the description change to uno:shrink/uno:grow.