I suggest it has to do with the demographics of game fans, the majority of whom are young & identify these "assets" as inherently sexy. Advertising their inclusion in games attracts more potential players, perhaps? It's also the case that very many games actually do feature big tits & arses. It's widespread & getting worse. Supposedly mature "Mom" characters suffer the most, with it being almost obligatory to show women with families in this way. There are a lot of otherwise beautiful sexy characters spoiled IMO, through having freakishly large airbags for breasts.
No notice of the laws of gravity or aging, realism or the disadvantages of being top-heavy, are taken into account. Many of the fans, through no fault of their own, have obviously never seen a mature woman naked & may think they all look like porn stars with artificial implants, that they've seen in fap mags & movies. Still there's no accounting for taste.