Bitcoin Core has shipped with a GUI since the first version. Originally this was a wxWidgets GUI, but in 2011 a move to QT was completed. Satoshi originally had plans to have a decentralized market place and even poker game inside Bitcoin, so including a GUI, which also had wallet and address book functionality, made sense from the get-go.
The motivation to continue to include a GUI with Bitcoin Core today is for accessibility. New users can access a best-in-class Bitcoin experience via a single software package. It’s not safe or realistic to expect users to download multiple programs and connect them securely into a software suite, just to use bitcoin.
It does not have to be the prettiest UI, but needs to provide the functionality to use bitcoin. It is possible to connect other frontends to Bitcoin Core, but they are connected via RPCs, and do not have the first-class interface (to the node component) that the bundled GUI has.