This is a spin-off question from this one "Determine radicals for simplified characters that lost their traditional form completely" where radicals for simplified characters were discussed.

Now, the question is, how were some characters simplified, sometimes so simplified they lost their original intention and form completely?

One example is:

葉 (leaf) became 叶, where 叶 is basically a mouth (口) and a ten (十) put together.

What's the rationale behind this??
And why were some complex characters not simplified at all?

For example:

(traditional : simplified : meaning)

單 : 单 : single, also a family name
嚴 : 严 : strict, also a family name
嚼 : 嚼 : to chew (here traditional and simplified are identical and yet it's complicated to write)

Note that the two 口 on the top of 單 and 嚴 were transformed into different forms when simplified.