What is the purpose of the block time/ difficulty in mining? I understand the point of mining is to decentralize away from allowing one entity to add to the ledger. What I don't understand is why it's necessary for miners to compete over who gets to add to the ledger. Why can't a random miner just be selected instantly and therefore bypass any block time? It would be just as random. What am I missing here?
Why can't a random miner just be selected instantly and therefore bypass any block time?


The problem is: how do you select a miner in a truly random way? If the mining process was just a simple random election, who gets to enter in that lottery? How would you stop someone from just running a million fake nodes, in order to increase their chance of being selected? 

These are the sort of problems that are solved by Bitcoin's proof-of-work mining. In a proof-of-work scheme, a miner must expend computational resources in order to 'enter the lottery', there are no 'free tickets', if you will. 

Proof-of-work also provides a mechanism of security: in order to change the blockchain history, an attacker would need to redo all of the work from the point of change onwards. To do so, they would need more computational power than the rest of the network (51% or more). A simple 'random election' would not include this 'real world resource expenditure', or the type of security it provides.