Why are through holes PLATED in PCBs?
In order for your scheme to connect the top and bottom layer, TWO conditions must BOTH be met:


The pad on the TOP must be accessible and must be soldered (separately).
The pad on the BOTTOM must be accessible and must be soldered (separately).


In very many cases the top pad of a thru-hole component is NOT accessible because the body of the component covers it. So that is not practical.

In MOST cases there IS NO component lead at all where you need to via from one side to the other. Inserting short bits of wire and soldering BOTH SIDES is simply not practical even for manual assembly not to mention automated assembly as virtual all modern gear comes from.

It doubles the effort to require soldering to BOTH sides of even a thru=hole component lead.  That takes double the assembly time, and greatly increases the chances of assembly error. It is simply not reasonable at any level.