Because most commercial appliances are too complicated.
The only things a 555 is useful for are short timing delays, flashing LEDs and making a speaker beep. Since LEDs with built-in flashers and speakers with built in beepers (called buzzers) can be bought fairly cheaply, this limits it to short delays (under a couple of minutes).
All of the above can be accomplished with an, PLA, FPGA or micrcontroller, if all a product does is have a short time delay the a 555 would be fine but most other products are more complicated. Complicated devices such as clocks, calculators, timers would be really, large, expensive and power hungry if they were implemented using discrete ICs (many 555s plus logic ICS) so manufacturers use a single programmable IC (microcontroller, PLA, FPGA) because it works out cheaper. Programmable ICs also are better because if the product needs to be changed, more often than not the IC can just be re-programmed - no component or PCB changes requireed.
I think the only product I've seen a real life 555 in was a timer relay and even then it was a CMOS 555, the TS555.
I wouldn't always recommend eBay, you need to be careful, it's often better to go with a 'real' local supplier even if it costs slightly more. You haven't filled your location in so we don't know what supliers to recommend.