My take on it is that it just isn't worthwhile to some companies at this stage. All that matters (in this day and age) is that a phone can make it through a full day. Some phones will go a day and a half and possibly two days at a push however most users will charge at the end of each and every day anyway. Therefore, if a 4500mah battery makes it through a full day the 'need' for a thicker and heavier battery doesn't really outweigh the cons.
My own case in point is my iPhone 12. It only has a 2850mah battery but it gets me through a day absolutely no issues and I'd much prefer that than having a hulking 5000mah unit that would weigh almost twice as much and take twice as long to charge. The latter is another reason generally as companies such as OnePlus would make it harder to market if their charging times are hampered by the larger battery. 50% in 20mins (or whatever it is now) looks good on promo material.