Why Bluetooth is so unreliable ?

I've used BT headsets of all types for many years since the Nokia feature phone era, on and off I've had problems with it. First I can't get a stable connection of both call and media playback, sometimes I get only one of them connected, and auto disconnect and reconnect...etc. I also connect to the Internet via Bluetooth DUN with an unreliable connection problem. Then I started to use smartphones mainly Android and some uncommon models but iPhone, and I used multipoint BT headsets, I used 2 devices one cell phone and one tablet of various models, one of the connections always disconnect when I moved out of range, and it won't auto connect again with I return, this problem seems to be happens on cheaper models of BT handsets most. Then now I am using Linux mainly Debian, it's 50/50 chances I got connected but the headsets give no sound, audio still route to the internal speakers, I need to dis-reconnect to make it works again, I am just sick and tired with BT, but it is the most common way get wireless audio from PC, why it's still so much of trouble over so many years ?