"We chat" is just to combination of multiple apps. There are many apps with similar concepts like messenger apps. You hardly find any uniqueness in Skype, WhatsApp, Viber and signal etc. but all are successful and have billions of users. They all trying to solve same problem with similar solutions. The only difference you find is UI design and some minor things. Similarly, Udemy, LinkedIn learning, Lynda and Edx offers almost same services. Do we need to find problem or can we solve same problem which is being resolved by competitors?
You ask a great question. Yes, it is possible to combine other services or create an existing service, and you might still succeed. But to be accurate, almost all of the companies that you mentioned did in fact solve a problem and/or have something unique about them that was unique enough to bring users.
For example: Skype was among the first systems at the time (being first...), Signal skyrocketed recently because they provide complete privacy which became a big issue after FB announced that they were officially getting all the info from users on Whatsapp (so privacy = unique), WeChat was targeted at the Chinese market at a time where the other big players weren't in China (so right time at the right place = unique).
Bottom line, you either need to solve an actual problem/pain, or be extremely lucky and be in the right time at the right place and have the right connections/community.