Most people who buy games regularly have a substantial number they have never started. Sales and low prices are the mechanism, and the pattern is avoidable.
Why it happens
A discount converts a decision about whether you want the game into a decision about whether the price is good. Those are different questions and the second is easier to answer yes to.
The habits that help
A wishlist rather than a purchase. If you still want it at the next sale, months later, that is a real signal. Most wishlist items quietly stop appealing.
One at a time. Do not buy the next until you have played the last for at least a couple of hours.
Check length before buying. Community-tracked completion times are accurate enough, and knowing something takes eighty hours prevents buying it during a busy period.
How to evaluate before buying
Watch unedited gameplay rather than a trailer. Trailers are edited to look good and a twenty-minute video of someone playing tells you whether the moment-to-moment activity appeals.
Read negative reviews specifically. Positive reviews tell you the game works; negative ones tell you what will annoy you, and you can judge whether those things matter to you.
Check the refund policy and use it. Most platforms allow returns within a time limit, and that is what it is for.
The subscription option
If you play many games briefly, a subscription library is usually cheaper than buying and removes the sunk-cost pressure to finish something you dislike.