keep in mind that the battery also runs the clock, which receives power even when the computer is turned off.
All modern portable computers (and phones) use Li-ion (lithium-ion) batteries, which discharge by themselves over time, even when they aren’t powering anything. According to to the article
Elevating Self-discharge - Battery University, Li-ion batteries self-discharge “5% in 24h, then 1–2% per month (plus 3% for safety circuit).”
Warm or hot temperatures can increase the rate of discharge. Older batteries discharge faster. And batteries that have been allowed to fully discharge also discharge faster.
You can’t fix it unless you can change the laws of chemistry.