lack of transactions in a block does not make it invalid, the miner may choose not to add any transactions (except the Coinbase transaction which states where the rewards should go to), however, most often than not, this isn't done with the intention of not wanting to add transactions, miners will start working on the next block right after a block is found, it takes a bit of time to deal with their mempool as they need to download the previous block's transaction list and remove all those transactions that were included in that block (failing to do so the block will be invalid).

So while downloading and working on the transactions, miners will start trying, if someone manages to hit a block before including any transactions, they will not hesitate to propagate the block, and that's how you see empty blocks.

Some pools don't mine empty blocks, but the vast majority do, there also seems to be a general "belief" that Bitmain mined empty blocks on purpose during the "block size fork war" attempting to halt transactions and make it seem like a bigger block is a must, other than that, empty blocks come as a result of what I previously explained.