There is always a new layer of abstraction available when it comes to building your own thing. You can still build a radio from a rusty razorblade but nowadays, building your own radio means buying a simple rtlsdr and setting up your own modem in gnuradio. The added layers of abstraction make it easier to do more with less time and effort. You can even design your own SDR but you'll still be buying VCOs and FPGAs since it's not really possible to do it with discretes, let alone homemade discretes. The economy of scale has greatly reduced the cost of using more advanced components. Someone could build a similar security system using all off the shelf wireless sensors without making a single solder joint. The challenge for a lot of DIY projects now involves the software, whether that means writing your own or hooking up various applications and services. Sure, you could build your own microcontroller board to talk to the sensors but that's a lot of effort you could instead apply to the software side of things.
