Why isn't PPP given due importance?
Nominal GDP is often more quoted in media and in politics than the GDP in PPP, but that doesn't make it be less important. In fact, as stated by 1muflon1 on the comments, economists typically like to measure differences between comparable things, so when comparing GDPs there are usual corrections done on purchasing power parity, as well as inflation. 

The PPP indicator has been present in literature for a long time, mentioned in the early 1500's, and translated to modern terms since 1918. It's regularly calculated by international bodies (such as the UN under it's International Comparison Programme). 

The GDP PPP is also regularly used in real life by institutions such as the OECD, IMF and World Bank to make recommendations on economic policy, and by traders to better assess when stocks and currencies might be under- or over-valued. This article on investopedia provides a concise overview.