Why did the convention "Co-founder" get started in startups rather than just saying "Founder"?

The United States is known for having its Founding Fathers, not its Co-founding Fathers. The term Founder doesn't imply exclusivity, rather the use of "The" vs. "A" makes that distinction - someone is A Founder of a company or THE Founder of a company.

So I'm curious, how did that industry get to the point of using Co- and why?

I suppose the question could also be asked this way, is it wrong to refer to a group of founding individuals of a startup as each being "Founder"?

During my previous startup, there were three of us - so each of us had the co-founder tag as each owned 1/3rd of the company.

However, later we realised that we all could be founders as we started the company together and it is fairly common in India to have more than 1 person as a founder.

Another reason for doing this was that the general parlance (atleast in India at that time) was that a founder is someone who starts a company and the co-founders are people who join later, have founder stock - but probably not as much as the founder.

Hence, we all bumped our cards to "Founder" so as to have better impact when meeting with potential clients and partners at trade shows, etc.

no founding father is alive today to say "I founded America". If you met one and didn't know about the others, you might think he invented America all by himself.

Since you can't hear that statement anymore in the 21st century, they are referred to as "The Founding fathers". A single unit.

4 people who start a company today are very much alive.

If one of them says "I founded the company" people can easily mistake him to be the only founder, which is clearly wrong.

Hence, Co-Founder.