No-one can keep their bicycle straight. Ive been riding for 40 years and still cant ride in a straight line. I do however ride with curves so large in radius that they appear straight.
A bicycle is inherently unstable and will lean to one side. You need to shift your weight and lean to correct the curve then you overshoot and the bike leans the other way. As you gain experience, the shifts in weight and lean become very slight and the bike goes more or less where you want. Generally it goes where you look.

You can practice extending the radius of your swerves. Pick an empty car lot and ride some slalolm around tin cans. Extend the distance between the cans and eventually you will approximate a straight line.

Most people learn to ride as young children when we are much better at learning and can pick up skills easily. We also spend a lot of time training these skills (called play). I used to spend hours with my neighbours riding up and down, in circles, over makeshift obstructions.