Why should we do anything that seem, detrimental in the long term despite it giving some fulfilments for the time being ?

Simple answer: stability is healthy.

Longer answer: Your brain can only produce so many "happy" or "feel good" chemicals. Your brain learns to associate the release of these chemicals when you do something beneficial or successful.

You only have so much you can produce each day. If you spend your day chasing whatever feels good in the moment, your brain won't be able to associate the "happy" chemicals with the types of success that life has to offer consistently.

If you do something particularly pleasurable Monday night (e.g. taking drugs or drinking), your brain won't have enough "happy" chemicals to motivate you to get out of bed for work the next morning. It won't have enough to motivate you halfway through your shift. It won't have enough to give you after you've done a long, hard day's work - it won't make accomplishments FEEL like accomplishments.

Since your brain won't be associating responsibility with gratification, you end up not being motivated to deal with responsibilities. You end up seeing them as meaningless and arbitrary - because your brain isn't making that positive association.

Worse, not only will your brain view responsibilities as without meaning, it may also end up associating toxic/unstable behaviors with meaning. If your brain is used to "feeling good" from gambling all day, you'll end up an addict of some sort.