Because not everyone needs 4K, much less at 60Hz, and this is cheap and small. A hub that provides 4K/60Hz would likely be more expensive, bigger, and/or lower quality.
Nobody likes to carry a bunch of adapters with them while traveling. If we could pick just one adapter, however, then this 3-port USB-C hub from EZQuest might be the best $30 USD you could spend on one.