Whats the problem in soldering the die directly to a pcb? are the bond pads too small that they can't be soldered directly to the board, similar to a LGA package? And then pour epoxy on it similar to a COB package. Like many chinese items have.
Depends on what you mean by "soldering". If you mean using a spool of solder and a soldering iron, forget it. You are off by a couple orders of magnitude. Your solder will cover 10-20 pads at a time. For "Chip-on-Board" (COB), they use the same microscopic bond-wire and ultrasonic welding that was used for decades for chip packages. Then they put a blob of epoxy over the whole thing to protect the die and bond-wires.
In more recent times a process called "flip-chip" is used where pyramids of solder are deposited on the bond-pads. Then the die is flipped over to face a PC board and the die and board are heated to re-flow the solder to connect all the pads to the board. In some cases, the raw die is left exposed. There are YouTube videos showing one of the flip-chips on the Raspberry Pi which turned out to be light-sensitive because it was unprotected and exposed to ambient light. People taking flash photos would inadvertently reset (or some other problem?) the circuit by the flash of light.
Modern CPUs are packaged this way. The CPU product is just a small PC board with the die bonded on top, and then a heat-spreader bonded on top of the chip. Frequently decoupling SMD capacitors are on the opposite "bottom" side of the CPU board to get them as close to the chip as possible. Then all the external connections are implemented with gold-plated pins (Pin-Grid-Array PGA) or lands (Land-Grid-Array LGA)