In a Venn diagram counting releases (especially Japanese PC), "hentai games" and "visual novels" have a great deal of overlap. However, "hentai games" includes a variety of games without a narrative focus, including gameplay-focused titles and sex sims. These are more familiar to some audiences than visual novels. "Visual novels" include non-adult releases, especially in English.
Which term is used also signals what is most salient or important to the speakers. To the casual onlooker, the presence of hardcore porn in this genre is more striking than the narrative format in use. To someone who doesn't appreciate the narrative format or is ambivalent about it, there's little reason to emphasize it (except to distract from the adult content). From my perspective, whether the games contain porn or not, it is striking that the vast majority in this genre focus on the interaction of the protagonist with pretty girls (or guys). Given that, whether the games contain porn or not is almost a technicality.