How do you determine the simplified structure formula?

Subject: Chemistry; Topic: Nomenclature; Example: Butane

I know that this is the "detailed" structural formula of butane

This is what the simplified structure formula (black) and the EVEN MORE simplified structure formula (green) look like:

My question: why don't you group the CH3s together but you do group the Ch2s together?

And what does the "simplified" structural formula (the green one) for ethane, propane, and pentane look like? (If you're too lazy but know it, it's enough if you answer the first question – it's more relevant)

Thank you very much