The reward for solving a block is 25 bitcoins, but the fees, which are added to that, are usually less than 1 bitcoin. Fees are only a few percent of miners' income, and the miners have several factors to weigh up when they decide how many transactions to put in a block

- The risk of making orphan blocks. A larger block may take slightly longer to propagate across the network, and that slightly increases the chance that another miner will generate a valid block during that delay and propagate it faster. When that happens, the first miner misses out on the block reward and fees, about $5500 today

- A miner could put no transactions in a block, and still get the 25 bitcoin block reward, but if they do that, they are hurting confidence in the bitcoin network, and devaluing their investment into hardware and skills and the value of bitcoins they own. If you drag the chart left to see the history at https://tradeblock.com/blockchain then you may see some miners who put zero transactions in a block or very few transactions, but it is not so common

Orphan blocks are an important issue in the debate about block size increases which the article linked to is discussing. Increasing the block size is likely to increase the rate of orphaned blocks, though there are proposals that might mitigate that risk.

The ratio of fees to block reward is likely to change in future, influenced by many factors including block size, and there are many competing theories about what will happen as that ratio changes, which run the gamut from doom to success. The block reward is automatically cut in half every 4 years, and the long term plan for bitcoin seems to be based on the fees increasing in value to eventually replace the block reward

One reason is because you would gain cents of fees but only in exchange for adding many many kilobytes of unsolicited commercial adverting to the chain. This material must then be processed by all nodes and maintained for all time, and in doing so burning the tolerance of others to participate in the system.

It's like asking why you don't litter when doing so would save you the few moments it takes to put something in the trash. The health of the system is more important to many than extracting a few cents more in fees.