Why do you use the past tense to describe the subjunctive mood?

[Rei]

I don't know if this kind of question is accepted or not, but let me try.
I have a question about the subjunctive mood.

"If I had one million dollars, I would buy a new house."
I know this means: "As I don't have one million dollars now, I can't buy a new house."

What I want to know is WHY you use the past (or past perfect) tense to describe what is not true now.
This sentence describes the current state, not past.

If there's anyone who can answer this question, I'd like to know the answer.
If this question sounds strange or stupid, please just ignore this.

Thanks in advance!

[Molly]

Because English verbs are not only related to expressing time. The "past" is used, in the main, to express past time, but is also used to express "unreal" or hypothetical situations and to express distance in social relations (politeness, unfamiliarity, etc.)?

e.g.

I went to the cinema last night. (past time)
If I were rich,... (distant possibility in the mind of the speaker. No specific time involved.)
Could I help you, Madam? (distance in social relations.)