Well, the problem is that if France builds a line of extensive fortifications along its border with Belgium, its effectively telling the Belgians that they are on their own, and that if they get invaded that's their own hard luck, neutrality be damned.

Obviously this is not a message you wish to send. Furthermore, Belgium itself could not build such a line of fortifications without compromising its own neutrality.

Instead, the France plan was to build the fortifications along the portion of their border directly shared with Germany. This would mean that the Germans would not be able to invade directly and force them to pass through Belgium. The French could then advance through Belgium, meeting and stopping the invasion before it reached French soil.

The plan was inherently sound and the Maginot line itself proved to be fit for purpose.

The problem was that the heavily forested Ardennes region, which formed the pivot point of the advancing armies, was commonly viewed as an unfeasible invasion area as the roads were too narrow and winding for sufficient artillery to be brought through.

As a result of this, the region was left with relatively light defences.

The Germans however, used the Luftwaffe to provide "flying artillery" and by making this lightly defended region the centre of gravity of their attack, where able to kick the door off its hinges.

The result of this was that the cream of the French army and virtually the entire BEF suddenly found itself wrong-footed, outflanked, and then cut off.

I'd recommend reading Blitzkreig by Len Deighton. Its a bit pop history and not entirely without fault, but it gives a good overview of these events and is very accessible.