Why do they put their speakers in the boot (trunk) where nobody can hear them instead of actually in the interior of the car? Is the idea to drive down the street with it open so they can startle or annoy everyone within earshot? 😕
Also, what happened to the old puzzled emoji? 😕 (Don't worry, I know this one.)
The short answer comes down to space and security. If you have a visible sound system, you have a much higher risk of your vehicle being broken into. When I had my Blazer back in the day my system was stolen in my parent's driveway. I learned after that and figured out how to mount my next system much better so even if someone broke into my vehicle they weren't going to be able to get it without some serious work.
The long answer takes acoustics into consideration. In a more sealed trunk, you're probably going to get an iffy sound, but for the most part, you can still hear it inside the cabin. I'm not an acoustical engineer so I can't give you all the science behind it, but essentially your subs need a bunch of air to reverberate and your trunk has that in spades. They can also use the trunk lid to deflect some of the sounds back into the cabin. To really make it sound good though, you need to have quality subs and a box with the appropriate volume and sealing on it. Most people, or at least people in the 90s and early 00s just wanted it loud, so they put two 15's in the trunk with the biggest, cheapest amp they could afford and let it bang.
Since I was a high schooler without a bunch of money, I opted to buy the biggest, cheapest stuff I could so I could be that guy in the school parking lot. I remember in my high school AP physics class, we used my truck for a demo when we did the section about sound. With a dB meter, the nerdiest dude in class sat in the driver's seat and I cranked the system. We hit nearly 120 dB, which is roughly the same as a huge boom of thunder.