Because the NANP (North American Numbering Plan) is a very complex array of codes that cannot ever conflict. Bellcore used to get paid a lot of money to ensure that all area codes and office codes never bumped heads as the NANPA (North American Numbering Plan Administrator). Since Bellcore is gone, this job was taken over by Northrup Grumman, then Lockheed Martin (or vice-versa) and now by Neustar.
At the base layer (end-office level), there are possible conflicts allowed due to overlapping local calling areas. For example, I might be able to call a "342" number in the adjacent "631" area code by dialing only seven digits. I might also be able to dial 1+516+342 to reach a distant community, still within my own area code that is a toll call. This is where the "1" becomes necessary to differentiate between local and toll calls. Once that "1" is dialed, your call moves up a layer to a toll office, which is where the IP and cell carriers reside.
Imagine that local calls, even between offices are carried by trucks on the roads. When routed to a toll office, these calls then travel by rail in bulk. Depending upon the area code (NPA), some end up traveling on planes. Regardless, all carriers have a somewhat redundant plan for transport so that if the planes can't fly, they can run more trains.
Since IP and cell carriers don't care about the difference between toll and local calls (everything is local to them), they never operate at the base layer. There isn't even a need to dial a "1" for toll-free numbers at this level.
There is even a uniform numbering plan for international calls. This explains why city and country routing codes vary so much in length. A call to London involves just a few digits for country and city routing, while a call to a town in say, Bangladesh might have only five-digit local numbers, so the city or country code is much longer to make up the difference. The last time I checked, the country + city + local number cannot exceed 18 digits.
To simplify the answer, high-volume traffic always operates on inter-office trunks straight to a toll switch. You rarely, if ever see these circuits (basically T1 or PRI) actually originate from an end office. If they do, then you are just paying extra for the telco to deliver POTS service. The general public usually only sees this in communities served with pair gain remote terminals or "SLICs".
Back to the local level: End offices can intercommunicate with each other within a region directly (LATA) without a toll switch being involved. They do this with local trunking directly between end offices. This is also where you see mixes of 7 and 10 digit dialing for local calls since these localized dial plans are maintained mostly by the LEC's and the state. As soon as that "1" is dialed, your call moves up to the next layer where everything is 10 digits as set forth by the NANPA administrator.
By the way, all LEC's, CLEC's and cell providers now contribute into a fund that pays for the NANPA to operate. This used to be a part of AT&T and the Bell System (hence the original name Bellcore). Once the Bell System was dismantled, it became an operation financed completely by a collective effort by all carriers who connected to the DDD network.
I have a feeling that I may be over-complicating my answer since I'm so passionate about this subject. Let's say that it is safe to assume that we will eventually become a national network of ten-digit dialing at some point for all calls. It was supposed to have already happened, but when pagers and modems fell out of popularity, the demand for additional office codes dropped off. This lessened the rush, but now that more and more local services are being provided by ISDN and IP, it's likely that it will be happening soon.
For some good information about this whole subject, you might want to check out
www.NANPA.com to get a better handle on how the whole thing is supposed to work.
I hope that I didn't wear you out in my explanation, but it is really not as complex as it seems. The NANP is pretty much the basis upon which IP addresses were established. Don't worry: I still haven't figured those out.