Why not? Because then it implies that a lot of other titles should be kingdom level.
Because the fewer vassals you have, the easier it is to manage something. This isn't really a problem with Kingdoms because Duchies are fairly regularly distributed throughout the map. So the growth of a Kingdom tends to accurately reflect reality, namely the larger that Kingdom is, the worse it's returns on holding that vassal are, since it's more difficult to manage large vassals and eventually you hit the vassal limit.
With an Empire outside of the aforementioned areas (HRE for example), it tends to get way too easy. Kingdoms outside of those areas basically represent what Empires are inside those areas. So you tend to have far fewer vassals then would historically be the case. For example, somehow it requires fewer vassal titles to manage the entirety of the UK and France then it would for just Germany in the HRE, despite that France alone was equivalent to Germany in terms of population and title density.
In the HRE at least kingdom titles automatically get destroyed if an Emperor holds them; I think at least that should be deployed everywhere since that would at least slow the abuse of Kingdom titles. As well presenting the annexation of entire kingdoms due to claims is a step forward. But I still think there are big problems.
