Facts are about 70% of proof, so why do assumptions prevail in discussions?

From what I can conclude, facts should remain the main point of any discussion.

In discussion, there are:

1. Facts which prove the topic

2. Assumption A which disproves the topic

3. Assumption B which disproves assumption A

4. Assumption that both A and B are false, leaving only facts

5. Assumption that both A and B are true, but negate each other, leaving only facts.

Going by number of options, there are:

1. Facts

2. Assumptions which disprove topic

3. Assumptions which prove topic

4. Assumption that 2 and 3 are false assumptions

5. Assumption that 2 and 3 are true assumptions, but negate each other

So we can see that by mere number of options, assumptions which disprove topic are only 1/5, where all other options support topic by upholding facts are 4/5.

So how does it happen that 1/5 assumption ends up dominating over 4/5, carrying conversation into meaningless rant?

Thats because 1/5 often gives enough doubt to actually be considered, and because it creates a hole in the story which facts cannot explain or cover.

What also happens is that facts get turned into assumptions.

Because every fact must be observable to be verified, it already follows that non-observable facts are non-verified.

And even observable facts have weaknesses, apparently depending on an assumption that what you see is really there and not an illusion.

Also assuming that conclusion from observation is correct and not itself an assumption.

So really, most discussions tend to go to area of assumptions.

But there is a way to counter assumptions, and that is by using lots of facts.

Even if opponent keeps using assumptions, there is nothing which says that those assumptions are correct.

But facts are correct, because most people assume that what we observe is what exists.

So facts do tend to help change people's mind, where assumptions always leave conversation at nowhere.

However, assumptions will always be the main weapon of those who do not have facts.

Proof of what?

Any theory that is 30% short of the necessary data, remains unprovable.

Therefore the 70% of factual data is not necessary to the argument, until the 30% shortfall is overcome.

There is no theory which can prove everything, due to many unknowns.

That creates space for assumptions.

Humans aren't as rational as you think

Yep.

Firstly, what is your definition of the fact? Do you mean the fact as it is understood by general populace in daily conversation, like statements which set in stone, or do you mean as it is understood by a scientist, like the observations and experimentally verified theories, or do you mean as it is understood by a mathematician, like axioms (of a system) and set of propositions deduced from given axioms. 

Depending on the topic, the term "fact" can be used to mean either of above but using them inter-changebly can lead to fallacies. As an example, if you posit the statement "sun rises from same direction" as a fact, as it is understood as general populace, then move to posit existence of dark matter as the same could be fallacious, even though both of them are "factually" observable. But one of is observed directly by human eyes, and other one is observed using instruments and calculations.

Basically, no fact is a fact, except God exists and ZFC axioms.

I mean observable things, things which can be verified to exist by observation.

One thing to consider is that there's not much argument with facts, so most of the space is devoted to the hypotheses.

Let's assume your definition of fact is factual. The problem is given any set of observations you can arrive at multiple different conclusions and some of the conclusions can be mutually exclusive. Can those conclusions be considered facts?

For example, we observe evolution, and the conclusion are multiple theories of evolution. Same with quantum mechanics, dark matter, psychopaths, and etc.

Mathematically, given set of axioms, you can with certainty claim all conclusions to be a fact. Can we expand such certainty to other fields? 

I acknowledge that, even the mathematics got a big problem in its fundamentals (formore info read about Mathematical Formalism and Godel).

"One thing to consider is that there's not much argument with facts, so most of the space is devoted to the hypotheses," - Greyparrot

Statement above is so wrong.

Conclusions can be separate from facts, and often are.

"Conclusions can be separate from facts, and often are."

Is above statement a fact? Or is it an assumption? The fact is above statement can be scrutinized at so many levels.