car stereos are loud because mobile amplifiers are a lot less expensive
higher demand = more market competition and more competetive prices
you can get 4000w rms @ 1 ohm in a car for ~$800, that same power for your home would cost upwards of $1500-2000.
And despite what most people believe, more car audio enthusiasts are well aware the importance of a proper subwoofer enclosure than you may be lead to believe by the goofnuts working at your local best buy.
The custom enclosure market is booming these days, as people are getting tired of their prefab sealed/high tuned ported/ and crapshoot prefab bandpass enclosures, and as people discover proper enclosure design, streetbass is getting lower, and louder with less power.
Any cars driving by that you can hear from your house are either throwing a lot of money at their cars, or have a pretty good idea what they're doing. Luck has very little to do with piecing together a kickass substage, nor does the number of drivers.
For instance, i'm only an amature at system design, and with 2000w rms, two 15" kicker L7's in 9 cubic feet, tuned to 33hz, i only managed to hit a 143.6dB on the termlab, while there's a guy by the name of mark potts, who hit 150dB with a single 15" type-r, and about the same power, as well as a guy named dominic iraggi who managed an ungodly 169dB with a wall of 6x9 drivers............