This is a very good question and the reason is actually a bit more subtle than already answered as the verb être used in expressions will be followed by the preposition de as in
être sur le point de faire quelque chose = to be about to do something
It is to do with the object being a verb in the infinitive and then you use what is called 'une préposition vide' ( an empty preposition) and it can be à but most often de.
What is my aim? to read as much as possible during the lockdown
The following example is from' Le Petit Grevisse'
But you could say the same thing using a noun instead of a verb :
Mon but est le maximum de lecture pendant le confinement
Le code de la route interdit l'ivresse
You could also say :
Mon but est que je lise le maximum pendant ...
Le code de la route interdit que l'on conduise en état d'ivresse
So it has to do with the use of the infinitive verb as an object.
Hope this helps!