in minified CSS why not combine and merge classes to be less size of code?

[webwamp]

why in minified CSS version, you not combine / merge classes to have less size of code?
for example Padding and Margin and ... here this example:

.py-0{padding-top:0!important;padding-bottom:0!important}
.pt-0{padding-top:0!important} and .pb-0{padding-bottom:0!important}

so you can easily do this:
.py-0,.pt-0{padding-top:0!important} and .py-0,.pb-0{padding-bottom:0!important}

in this combine you have very minified and less size codes, so why you not do that? anybody can explain to me?

[XhmikosR]

We have experimented with such optimizations and the gains aren't too big to justify the potential breakage.