Daylight photo reconnaissance was a nightmare for Luftwaffe crews, given the aerial dominance over the Channel by June 1944. Night flights were more possible, but still faced ferocious opposition by Allied night fighters--some of which were equipped with their own radar units to track the opposition. Mounting hourly flights would have been an unsustainable operational tempo, given how starved Germany was for more pressing concerns like fighter defense for their cities and infrastructure. Germany was short on pilots and planes, especially experienced pilots. A few hours' warning was not worth the potential cost of repeated recon flights over the Channel.

As others have mentioned, the Allies had superiority in the air over the channel, any reconnaissance would have been very difficult. The Germans also had other ways of getting information about a possible invasion:

The Germans knew that there was a threat of invasion, they started building the so called "Atlantic Wall" in 1942, much was made of this but in reality it wasn't nearly as impressive. When Rommel was put in charge of the wall he found it simply didn't exist in some places, in others it was n't exactly impenetrable.

Logic said that you'd launch an invasion where the crossing was shortest, thus the Pas de Calais area was the obvious choice. Allied leaders thought it to too obvious though, also through the painful experience of the Dieppe raids they knew that attacking a port/harbour wouldn't work as they could be defended too easily, so this limited landing zones in Pas de Calais further.

So the Normandy area was selected, however in the weeks (months?) running up to D-Day the Allies would bomb/attack a large area from Pas de Calais down to Normandy so as not to give the game away.

Furthermore, the Germans had no idea that their UK based spy network was compromised. They trusted all communications they received and the Allies were very clever in using this as an effective weapon against Germany - disinformation and outright lies were fed to intelligence services on the German side to help the invasion succeed.

One of the most important agents around this time was nicknamed "Garbo", real name Joan Pujol Garcia. He was a Spanish national who had gone to the Allied embassies and offer his services, they turned him down, several times.

Undeterred he went to the Germans and posed as a Spanish government official who would make trips to London, claiming he was pro-Nazi and offered to spy whilst in London.

They were only too happy to have his help. He didn't go to London but Lisborn, and basically made up all sorts of "intelligence" to feed to the Germans. Given he had no military background (he was a chicken farmer) it is quite surprising that the information he made up was plausible to the Germans - indeed when the Allies found out what he'd been up to they were quite shocked at just how well he'd done.

Eventually the Allies took notice of him and shipped him to England, where he continued sending messages to the Germans, albeit now constructed by the intelligence services.

The information he gave to the Germans was backed up with other "evidence" paraded in front of the Germans to make it seem plausible.

The basis of the entire feint was that the Normandy attacks where just a diversion and the real invasion would take place in the Pas de Calais. This worked - the Germans held forces back, giving the landing forces vital time to secure footholds on the French coast.

Garbo's story is quite amazing, there are several books/TV programs about the D-Day intelligence effort which will give more detailed information. He played a vital role in deceiving the Germans about the invasion and from what I read was completely trusted by the Germans.

At that time, the German's were engaged in the East with the Soviet Union. This had priority over anything else. Also, the Luftwaffe (German Air force) suffered TERRIBLE casualties during The Battle of Britain. Not to mention, imagine the logistical cost/effort it would take to monitor Northern France (granted, they had an idea of where the allies would land, but they weren't certain).