not enough warplanes (Germans have two luftflotte, 2nd and 3rd, concentrated on West, and they have more modern planes than all French air forces).
On 2nd September 1939 Luftflotte 2 had the following serviceable aircraft:
229 Bf 109
172 He 111
27 Do 17 (in recon role)
30 Hs 126
8 He 46
Luftlotte 3 had:
238 Bf 109
24 Ar 68
273 He 111
77 Do 17 (all but 3 in recon role)
34 Ju 87
69 Hs 126
19 He 46
3 He 45
Western Front total:
491 fighters
549 twin-engine bombers
34 dive-bombers
129 tactical reconnaissance (single-engine)
A large protective force but perhaps not massive enough to overwhelm the French air force in a counter-attack but defensively the strength of the fighters would have been enough to make attaining air superiority a very difficult task for the French. Nor indeed a large enough bomber force to worry about any 'knock-out' blows against cities.
About 180 of the Bf 109s were assigned to the newly-formed Zerstorergeschwader as there were no available Bf 110s, 4.48% of the serviceable fleet above are still biplanes, there is very little tactical ground attack capability.
This is of course a single snapshot, it offers little relation to the improvements made by both sides in the OOBs from September 1939 to May 1940, indeed there is no possible impact from British and French wartime production, no time for any impact of US-deliveries already made. So qualitatively all three nations are on the back foot to some extent before the cheque books started waving around (for example even aircraft would be ordered from Italy by Britain and France).
Also it would take France at least a month if not more to really make a good ground offensive effort. In some ways you can praise the fairly rapid response that brought forth the Saar Offensive within days of the war breaking out, but it was probably as much a lack of in-depth preparation as much as timidity that saw it quickly peter out.
I think if we are to be honest, Britain and France didn't want to get its hands dirty.
They thought it would be relatively easy to let Poland with its large army to take the brunt while they thought up some solutions, probably hoping after some battles and losses that Hitler might think again and come back to the negotiating table.
Reinforcing Poland or sending war materiel was practically impossible due to its geographic location and any Anglo-French naval blockade of Germany would take time to have effect and Germany's efforts at autarchy were already known to some extent. Britain probably spent more time thinking about how to bomb Baku despite never declaring war on Hitler's non-aggressive ally who happened to also invade Poland. As Del_ has mentioned above, despite all the bluster of the inter-war bomber barons, they made conspicuous little effort to put their plans into operation - a woeful lack of intelligence on targets not helping. In addition the civil governments didn't want to trigger a response and invite their own people to be bombed (not a good vote winner if the war did 'blow over' after a few months. Also why invite legal claims for damage later (again, if it all 'blew over' after a few months).
Probably no-one foresaw quite how Blitzkrieg would unfold. A lot is said about combat experience in Spain, but it was a long and bloody drawn out war. There never had been an invasion and collapse like that in recent European military history on such a scale. The theories of Fuller, Hart and Guderian were just that - nobody seeing the fleets of relatively small tankettes or even the relatively puny (by later standards) medium tanks of 1939 would have necessarily believed them capable of such inflicting such damage. A lot was written, and observed in Spain, of bombing against urban areas but very little public exposure was made on tactical ground support airpower.
Neither the Chamberlain or Daladier governments had a plan, they just felt they had to act and fulfill their threat to Hitler so all they could do was declare war and hope it got sorted out. Even Churchill's government in May-September 1940 had no real plan, just a bluster of "we will never surrender" and hope that no Plan B was required, or even thought of what Plan B might be (in Churchill's mind Plan B was how to drag the USA in, but that wasn't necessarily a sound back-up plan in 1940). The rapid defeat of Poland made all their assumptions void and the 'Phoney War' reflects the paralysis on both sides struggling to figure what to do next and how to force either side out of the war (Churchill's meddling in Norwegian waters causing serious blowback when Germany pre-empted the Allies). Perhaps it was some semblance of guilt that persuaded Churchill to send some of Wavell's units to Greece in 1941 to offer some practical support for an ally - ironically dooming both Greece (not enough on the ground to make a real difference) and losing to chance to have completely secured all of Italian Libya by May 1941 - which was probably a bigger a blunder than the loss of Malaya/Singapore/Burma. Had Northern Africa been swept clear before the Afrika Korps had left Italy the entire course of the war would have been drastically different (for good or bad).