There's only so many things you can do with just those 2 characters. I think the appeal for this type of game is that the main character is supposed to be tempted by other girls. So while I agree that the focus should be on Tracy and Lisa, spicing up their scenes for later needs additional girls for some lesbo and threesome stuff and for additional conquest options for MC.
Adding scenes with other guys holds zero appeal for most people who like harem games unless it's wife swap or swinger type content where both the MC and his love interest are getting some action on the side. I think most people playing this type of game want either a harem type game where the MC fucks everything in a skirt and convinces the primary girls to do threesomes with him, or they want cuck content where the primary girls are cheating on MC. There's very little crossover.
