I understand cultures are different, however, I don't understand why nobody bothers.
In French culture, it's considered vulgar/rude/distasteful to talk about money in public, so the idea of itemizing the bill and everyone paying their share only isn't really practiced. Instead, depending on the number of people attending, the group will talk about who's going to take care of the bill for everyone else. In this way, the mentioning of money, and/or the act of being "frugal", is avoided. If the group is large, then people usually split the bill evenly, regardless of who ordered what.
If you're hanging out with the same [smaller than larger] group of people, then friends will take typically take turns paying the bill.
Be careful in how you ask your friends to dinner though.. if you're always directly inviting the group by using a phrase like
Je voudrais vous inviter au restaurant. [I would like to invite you to the restaurant.]
then it's assumed that you're going to take care of the bill. Instead, you should perhaps use a more casual phrase like
On se fait un resto ce soir? [Are we going to a restaurant tonight?]
In this case, it's unclear who's paying, and so, there's good chance that a conversation will be had in order to determine who should pay. And, since you most likely paid the time before, if they're actual friends of yours, none of them should look to you.
Without knowing the verbiage of how you've been asking them to dinner, I can't say with confidence if you should let this get to you or not. I do recommend though that you keep an open mind about it, and regardless of the outcome, take this as a learning experience for a culture that you're still adapting to.
If, in the future, you start hanging out with other people, and those people don't act like the current group [and are more generous when it comes to the bill], then I think that will be most revealing.
Pro tip: If you want to avoid this situation altogether, then, from time to time, tell your friends that you have to leave early (just make up some excuse), but, before doing so, go to the waiter/waitress and tell them you only had "this" or "that", and make sure to leave that amount of money on the table just before you head out.
Or, if you don't want to lie to your friends, simply say that you're going to the restroom, and while you're away from the table, pay the host your portion of the bill. When it comes time for the bill, simply tell your friends that you already took care of your share. Depending on how you say this, and, if your friends expect you to pay, they will either be 100% okay with this, or, they'll be a little passive aggressive about it, and/or change their behavior in the future.
But, again, it largely depends on how you've been asking them..
