Fobs are extremely common in workplaces. Some cars are starting to have them built in as well.
Why are new residential homes still being built with metal keys like it is 1920?
1. Homes are not mass produced
2. People who build homes learn it as a trade
3. Not a standard feature
4. No good off-the-shelf products
Homes are built by people who are good with their hands. I worked for a homebuilding contract for awhile in college. Most homes are still built with highly-flammable wood, which means your house burns down if it catches on fire, instead of newer materials like insulated concrete forms. So there's the traditional, hand-me-down generational knowledge, and electronics usually aren't a big part of that. Cars are massed-produced by the thousands; homes are usually only mass-produced if they're trailer homes or prefabs.
Plus, it's not a standard feature on homes, and there's really no good off-the-shelf products available, partly because there are a lot of issues involved. The best system would have the following features:
1. Electric line for constant power
2. Battery backup for power outages
3. Manual backup (mechanical key) for power outage
4. Easy system to "change keys" (ex. if an RFID card gets stolen)
5. Smarthome integration (Bluetooth & Wifi support for smartphones & computers) with remote access
So now you have the following problems:
1. Run electric to an area that usually doesn't have it. A lot of doors have side glass windows to peek out, so you'd have to figure out a way to get the power through that. You'd really want it on the door frame portion as well, so that you could buy a standard door and retrofit the electric lock in the standard hole, but have the battery & electrical connections permanently-mounted in the frame.
2. User has to replace battery. This is an optional feature, but if you're going to pay for an electric door system, then you probably want something that can handle being out of power for an hour or two and still work. You'd still need a mechanical key for emergencies, but again, if you're paying for an electric system, then you want it to work and not have to carry a traditional key with you. In a car's RFID system, the battery is constantly being recharged, so it's rarely an issue, which is why it "works".
3. You'd need some kind of easy re-authentication system to pair new or replacement keys, like if a card gets stolen or something. So you'd need a re-order kit, and you'd have to make sure the company was strong enough to not go bankrupt after a few years so that you could still get support for the batteries, replacement RFID tags, etc.
4. You'd need a programming team to keep the system updated, if you wanted smarthome integration - iPhone/Android/Blackberry/Windows Phone apps, a computer interface, integration into control systems like Crestron, Savant, and Control4, that type of thing. And because those systems are constantly being updated, you need to pay programmers all year long to keep the system updated, as well as fix bugs, install security patches, and so on.
There are a few nice ones on the market or coming out soon:
https://lockitron.com/preorder
http://www.kwikset.com/Kevo/default.aspx
http://www.gojiaccess.com/shop-goji.html
http://www.getdoorbot.com/
http://www.gokeyless.com/
But, it's a difficult problem because it really requires ongoing company support to be viable - software updates, batteries, replacement keycards, etc. With a mechanical doorlock, it ALWAYS works, and if it doesn't, you just call the local locksmith to fix it, or swing by Home Depot and fix it yourself.