Yes, there is a reason. The internet doesn't have the capacity to send an infinite amount of data at one time. The more data that has to be sent at a given time, the more packets are delayed or lost and the slower it moves. If a server is needed to store larger files, it becomes physically larger and more expensive.
I don't know how old you are but less than 15 years ago, the largest hard drives that were offered on consumer-level PC computers was 1GB. You think this is a convenient file size and it's larger than hard drives used to be.