Simple answer it poses high securitty risks to corporate network.

will give you an example...

Say User X who works in finance dept Remote access VPN from his home into his work corporate network that allows split tunneling, at the same time User Xs home firewall has a rule to allow RDP access from anyone on the outside to his machine.. becuase the corporate split tunnel policy User X internet is not going through corporate firewall, therefore User YY who is a finance hacker expert RDPs into User X machine and he/she will be able to look at finance folders while User X is VPNed into corporate network.

If RA were full tunnel vpn, User YY finance hacker cannot RDP into USER Xs machine because it will required to cross Corporate firewall .

Easy right?

Regards