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The reason why is that even if you are on the winning side sometimes, a lot of factors ensure that you’ll be on the 49% side every now and then. And then it is in your interest that your rights are defended. This is not some new or particularly deep insight. It is the basis of political liberalism.
But it is only technically correct, if tyrants regularly agree to cede the power to new tyrant without bloodshed or even a tantrum.
It's exhausting for the other 7 billion of us who want to talk about literally anything else
For instance, this is probably the worst part about being in the UK while being from the US. It seems rather difficult for people from places very closely tied to US politics (culturally, linguistically, politically, and diplomatically) to not redirect a conversation about any other country to the US especially if someone from the US is present.
Universal suffrage is an ideal entirely reliant on how the denominator is defined. Delineating the polity (i.e., polis) is an institution in democratic exercise--we traditionally punt this question to that of citizenship.
(On the opposite, you have countries where you can vote in local elections even if you do NOT have citizenship.)
Of course we do. Because it’s convention. But not everyone within a border geometry is a citizen. And why a border geometry is what it is usually relies on other questions.