why should you test only one variable when doing an experiment?

By isolating only one variable at a time, its effect on all the other variables can be measured accurately. More specifically, it allows the experimenter to observer, record, and measure a discrete phenomenon without any interference.

Because if u test more than one variable, u do not know which one makes the experiment work
For example:
i have a seed, i wanted to know wether sunlight is a factor which helps it grow, so i put one under sunlight and one in a dark room and pour same amount of water into boh of them. That's testing one variable. If you test more than one variable, u would be putting one under sunlight and provide water, and the other one in darkroom and give Coke to it. And the result would be the one under sunlight and provided wth water grew more than the other one, but now, u dunno which variable makes the seed grow more than the other, is it the water or sunlight?

Hope u get wat i mean

It is really difficult to try to use just one variable in an experiment, but this has to be done.
If you use 7 different variables, how do you know WHICH variable caused the results?
Example, you have a chemical formula with 7 compounds or elements. You mix sulpher, lime, carbon, charcoal, sugar, nitro cellulose, and sand, and want to know if you change the AMOUNTS, what you will get. If you change all of them, all the time, and try each mixture, you might get something that does something, but you may not learn WHAT ingredient is doing what and why. By starting out with 6 the same, and changing one, you can determine what affect ONE ingredient has on the entire mixture. Eventually, you will learn WHY certain ingredients are best in a certain amount, and what exactly they do in the mixture.
If you changed everything in every batch, you would get different results, but you would not know which ingredient did what, or what quantity had what effect. you would have to do thousands and thousands of tests to find enough data to plot a chart that had enough information to evaluate just one ingredient - better to do 10 experiments with only one ingredient changing, and then you can draw a chart or graph, and know what the effects are.
Then change the next ingredient, and make a chart for it, etc.
PS, if you started this experiment with the accidental correct mixture with the correct amounts, you would only have to do this once, since your lab would not be there after the first experiment...

Because the other variable is shy and tenses up when its test time. You gotta be more compassionate towards variables you know.

If you vary 2 variables between the first observation and the second, you don't know if the difference observed was caused by the first variable, the second variable, or a combination of the two.

it provides more accurate evaluation of data. one variable ensures that it truly is the variable that causes what you are experimenting.