Except Apple could have made it easy for people to replace batteries, knowing the batteries won last past certain number of charges, they nevertheless insisted on a design that makes replacing them a huge hassle.
Adding ease of battery replacement adds failure points, weight, thickness, complications, openings for dust and other pollutants to the electronics. The batteries have to be encased in plastic, and a connector has to be engineered to be easily handled. . . for a battery that can catch fire if not handled correctly and requires proper disposal. It's an engineering and design decision.