You might understand that some helpers may become disgruntled if they put time into developing a solution here only to find later that a solution had already been achieved elsewhere. They may then be less likely to help you next time.
You may also want to consider whether cross-posting actually increases your chance of getting a quick answer. Some (but certainly not all) members, because of the point above, tend to skip questions that are cross-posted because they don’t want to bother going to the other forum(s) to check if it has already been solved. It can also become rather messy trying to keep up if the threads are active in multiple forums at the same time.
The advice I usually give members is to pick the primary forum that they wish to post to, and post their question there. Give it a day, and if they do not get any replies, then they might want to consider posting to another forum (though if you do not get any replies, you may want to make sure that you have asked a good, complete, concise question -- we have a thread to assist with that: https://www.mrexcel.com/forum/board-announcements/127080-guidelines-forum-use.html). Most people will see that approach to Cross-Posting as reasonable.
What often gets frowned upon is what some of us refer to as taking a "carpet bomb" approach, where a person will post the same question across 3 or 4 Excel forums within a matter of minutes. What people also need to consider is that many of the members who answer questions are on multiple Excel forums, so they see what is going on.