Because USN vessels were already escorting convoys, and U-boats were not allowed to attack them.
So what do you think is happening after Pearl Harbor? My guess Roosevelt extends the neutrality zone all the way to the UK, more USN vessels are going to escort convoys, more US merchants are to transport goods to the UK (and USSR), and at some point too many of those are getting sunk and the US declares war . So Germany can either shelve the U-boat war alltogether, or accept that they're going to get into war with the US at some point. And if it's the latter they might as well do it now, because delaying it plays in the hands of the US.
Now I'm more of the opinion that Germany shouldn't have bothered with the war in the Atlantic at all, because they were never going to win it, but that's another topic.
I once did a back of the envelope for US extension of the Neutrality zone/s. This had not been a static line OTL, but had been extended several times . The last extension I recall was the USN occupation of Iceland. Hitler had become aware of the US intent sometime in April or May, and was fairly pissed about it when the marines landed in June. But, he was deep into the preparations for the Barabarosa operation and tabled a DoW til later. (WI he had hammered the table harder and made a DoW on the US in June 1941?)
So, if there is no German DoW in December 1941? Extending the Neutrality Zone might look like this?
1. Negotiate or force the Portuguese to accept US occupation of the Azores & Maderia. Now there's no chance of the German submarines refueling there, and US neutrality patrols can interdict any they spot in those waters. OTL the US had a plan on the table for this, but the political situation did not justify the cost in 1941.
2. Extend the Neutrality Zone to Liberia & Sierra Leone. Both hosted US air bases during WWII. Supplements the Azores as a ASW base.
3. Extend the NZ to Angola. Same situation was #1. It might even be done as part of #1
4. Heres the big one. Negotiate with the Vichy government to make the Africa colonies a Neutrality Zone. Technically the French were neutral. More to the point they were pissed about the Germans giving the Japanese their blessing to occupy French Indochina, and they feared the British would invade the Africa colonies one by one. Allowing the US to 'protect' the French colonies is better than losing them to the Japanese & perfidious Albion, or the Spanish & Italians. The US would have the forces to do this by October 1942 if not sooner. At this point Hitler is likely fed up and goes for all out war with the US, but he is worse off than back in December and the US has a better ASW force built up.