Because one big mistake many people make is writing down the name of the wallet that the seed phrase becomes to, and as such making it very easy for anybody to have access to your fund automatically. But if we can write just only the 11 Seed phrase without a clue which wallet it becomes to, it will be very hard for anyone to have access to it very quickly.
If you know what wallet you use for wallet creation, you don't need to write it down. Because backups are for recovery when your memory can not serve you, then writing down a wallet software name is good practice.
Like me, I don't use any other wallet than Electrum wallet, I don't need to write it down. If you use many wallet softwares, it will be a mandatory part of your backup but if you are careful, you can save it at a different place than wallet seed.
With wallet seed, you can figure out what wallet software use to create it, but if you lose wallet seed, you lose your coin.