Because they offer you to save only during free-roam.
Well that's generally a staple of Games on general, in any kind of game you cannot save while a conversation (from Maniac Mansion to Fallout 4, in ages and genres), while a cutscene, etc.. the feature of saving Inside a cutscene/conversation it's something usually exclusive to VNs. The ability to transform vn's into freeroams of some kind it's just something good that renpy has. That's said, it's just the classic point; that every engine is a tool to make some kind of work.
to the OP;
Same reason, it's between reason that people who use rpgmaker just wanna make a game with a more gamey feel (rpg or not) and people who use renpy a game with a more VN feel (VN or freeroam or whatnot). Quality of content And gameplay is up to him.
That's why I always check Unity/Unreal/GameMaker games because they feel more unique (unless they're just using a VNtemplate, then they're not relevant to this), the work to put in place the engine to do what you want to do with it goes for a personal work and if the game is good goes for a nice and new experience.
Anyway, about quality of content is the same as SFW games; AA and AAA tittles on less measure because theres money spent, but well, just a little percentage of them are good games or fully enjoyable.