Well, those are the reasons why PES 5/6 shine (but I can go even backwards that includes also PES 3&4), where the modern games are not even close:
- fluid & responsive gameplay
- smart AI that can approach & adapt differently
- unpredictable moments on the field while manipulating the ball / tangible contact between players / lots of diversity in goal scoring opportunities
- keepers are acting naturally and not over exagerated during animations
- in depth tactical & individual player postioning by unique stats or condition in the game plan
- risk/reward mechanics that counts by changing formations during matches or picking subs
- stamina depletion that can destroy your pacing if you're pressing always the sprint button
- fatigue & injuries are crucial in a long run during cups or entire seasons
- very fast menus to navigate or to transition into matches / on point UI (no pink/purple neons blink-blink)
- there are rewards to unlock in the PES-Shop while you're playing & enjoying the game (players, classic teams, stadiums, music, etc.)
- and lastly but not last, the modding community:
Those are my all time patches favorites, but you can experience classic PES 100 times differently by adding some prefered flavor to your vanilla with fully modded conversions made by this great EvoWeb community: classic encyclopedia patches of the past decades back to early 30's, all posible seasonal patches made until 2024, or mini patches that can offer an entire WC/EURO experience, fully made with care.
Those previous bullet points in my opinion are what makes those PES versions timeless masterpieces, packaged with such a deep replayable value...