It's a legitimate question. Airplane crashes are terrifying: oftentimes a single crash claims the lives of hundreds of people. If passenger airplanes are equipped with a host of other equipment meant to save lives in event of catastrophe - inflatable rafts and flotation devices for water landings, oxygen masks for cabin depressurization events - then why aren't parachutes available for passengers to safely jump from doomed aircraft as well?
The answer is simply that parachutes on commercial aircraft are hopelessly impractical. The high altitudes and speed at which commercial aircraft fly make jumping dangerous. There are tremendous logistical issues associated with storing and periodically inspecting enough parachutes to accommodate an aircraft's passenger capacity. Parachutes require training to use (there's a reason why a beginning skydiver's first few jumps are tandem jumps) that the typical civilian passenger doesn't possess. The organization required to get hundreds of passengers to strap on parachutes and exit the aircraft before it crashes is, to say the least, mind-boggling.
Furthermore, instances were parachutes could make a difference are vanishingly small. While there may be examples of flights that were doomed but which remained airborne long enough for them to conceivably be evacuated, if such an option were available - Japan Air Lines Flight 123, United Airlines Flight 232, and the hijacked aircraft of 9/11 come to mind - the vast majority of plane crashes occur during initial take-off or final descent and landing. These are situations where parachutes would not be usable due to the plane's altitude, and where the reaction times needed to get passengers to don parachutes and exit the aircraft simply do not exist.
There are many technical reasons for airlines not offering their passengers a parachute in the event of an emergency bailout. However, one significant reason airline passengers don't have access to parachutes is commercial air travel is our safest mode of transportation. The statistics have supported this conclusion for many years, and it's essential to keep it in mind before 'jumping' to conclusions about the need for parachuting passengers.
It's worth remembering that commercial air travel is remarkably safe; so much so that you are eight times more likely to die in a car crash on the way to the airport than you are on the flight itself. The United States, at least, hasn't experienced a commercial aviation disaster claiming over 100 lives in over two decades. And in the rare event that an incident does occur on your flight, the National Transportation Safety board estimates that you have over a 95% chance of survival.
So enjoy your flight.