Even though the phone's may look and perform similar they do tend to have newer chipsets i.e Exynos or Snapdragon in them to keep up with new software demands and features along with other parts inside along with improved glass displays back and front and new materials.
Phone's only receive so many full software updates and Security updates over it's lifespan.
Samsung also need to keep up with the competition and market demand from consumers.
People want the newest phone available sometimes even if the newest phone isn't a vast improvement over it's predecessors.
They want different sizes to suite their tastes and different models with materials such as plastic backs or glass backed phone's to suite a person's budget.
Networks want new models to push and advertise to draw in new contracts and upgrades.
Supply and demand is strong in this sector @nelly13
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.