Why a highly concurrent system is mostly asynchronous?

The actual problem in synchronous systems is the waiting time between the client and server where both have to wait for the process to complete. In a system where millions of requests are made per second, they usually take the request and process it asynchronously with messaging systems like Kafka or any MQ's. A gateway client may wait for the response event to respond to the user, but the actual process happens in async (maybe) in the internally distributed systems. So there are two questions I would like to ask -
1) Is my concept correct about high concurrent systems?
2) Is there any way we can make this kind of system synchronous? (internally)