Intervention did occur, in the Saar Offensive immediately after the invasion of Poland on the 7th of September. It consisted of 40 French divisions (as well as 2000 tanks and 5000 artillery guns) crossing the Rhine valley and attacking the German 1st Army, which consisted of 22 divisions, 100 artillery guns and zero tanks. They vastly outumbered the Germans and yet the offensive was halted after less than a week. Why on Earth would they do that you ask?

Slow mobilisation, out of date tactics and a general unwillingness to fight are considered of the main causes of the failed assault, the French commanders believed infantry advancements must be supported by artillery barrages (as per WW1), contrary to the Blitzkrieg employed by Germany. These 5000 artillery guns all had to be brought out of storage and deployed to the front line. Note that the Germans did not defeat the French in the conventional sense, rather the French voluntarily retreated after a series of minor skirmishes.

Had they pressed on against the defenceless 1st Army, the war might've been over a lot sooner. The French government seemingly wanted to minimise provocation of Germany, fearing a counter assault equal or larger in strength (they got that anyway). Generals with their minds looking back 20 years were hesitant to have their armies leave their fortifications, slowing advancement to a snails pace.

It was a collosal failure by all metrics, and it inadvertently condemned the French to 4 years of Nazi rule.

The British did not attack as they would've violated Belgian/Dutch neutrality.

On mobile so I will post sources when I have access to a computer.

Attack the western past of Germany when the Germans reoccupied the Rhineland?