Because yarn completion does

1. Execute yarn run --json
2. Parse 1. output and collect packageScript(scripts entries in package.json) and binCommands(scripts under .node_modules/bin) by sed and tr command
3. Construct completion list from static list(yarn subcommands) and 2. list

As you know, we can use yarn packageScript instead of yarn run packageScript and yarn some-command instead of ./node-modules/.bin/some-command. So yarn completion should do above procedures.

I suppose faster completion commands uses only static list for completion.