I really think the reason why it's taking so long is De Ligt wants to make the best possible decision to help him develop. He's a down to earth guy, I don't believe it's about the money at all. He said so himself. Also, Raiola isn't his official agent, it's Barry Hulshof, so I'm not sure how they are working things out etc.
There are lot's of factors to take into consideration when you are that young and the world is at your feet. Make the wrong decision and you could very well fail to reach your potential. Look at Mbappe: chose PSG, allegedly for the money, and wants to leave already, probably because he'd wished he made a different choice a year ago. Matthijs doesn't want to end up in the same situation.