Sure glad that's not why they did it because you're right, that'd be kinda stupid. That's not why they made a secondary layer, though. They made a secondary layer so transaction throughput can grow exponentially while maintaining the security of the blockchain without significantly impacting fees or requiring the blockchain itself to become proportionally larger.

That last part is the real motivation. The goal is to above all else, remain decentralized. That means the average user needs to be able to run a full node capable of verifying any transaction it needs to. To do that, the blockchain can't grow too quickly, or people will get forced out. If it grew exponentially faster as transactions grew likewise, nodes would eventually centralize in fewer and fewer hands until someone could exert control over the network.

The blockchain is currently something like 650-700 GB, which is a lot, but most people can manage it, even if it might be pushing it for poorer regions. Without the lightning network and with substantial user growth, the only option is to increase the block size, and to achieve an actually usable capacity of strictly on-chain transactions, you'd be looking at sizes on the order of hundreds of TB or pushing into PB territory. Nobody would be able to store the blockchain without a dedicated server rack. Not a single server, a whole rack. It'd costs thousands and thousands of dollars to run a node. Instead, we acknowledge that you purchasing a pack of gum at the convenience store doesn't need to be immortalized on the blockchain and use the lightning network to secure your transaction without having to create a permanent record.