Because you don't have to sell all 100 1 oz coins at one time or even all at the same place - as opposed to 1 100 oz bar.
Do you mean the kilo Lunar? Milk in singapore comes in 1 litre carton which weighs about a kilo. A litre is 0.26 gallon. For some reason, the kilo coin weighs more than a 1L carton of milk. I think it's psychological. When an object is denser, it feels heavier even for the same weight.
I just checked a kilo coin and it contains 0.27 gallons of silver. This is likely due to the mint producing items that are slightly overweight to ensure they comply with sales laws and regulations. Packaged food items must state their entire nett weight including packaging to ensure they comply with sales laws and regulations.
If one is serious about investing and in later stages of life..... 1,000 oz is only $15k usd but weigh 32kg..... 10,000 oz is $150k usd but weigh 320kg And a kilo of silver isn’t small