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That's not true. Most conversational platforms do use a separate "trigger word" subsystem which has a much smaller vocabulary and is optimized for the low power, always on scenario. But the trigger words aren't hard coded - for example on the Motorola X series you could (can?) customize the trigger phrase: https://motorola-global-portal.custhelp.com/app/answers/inde...

I think "[keyword]" is meant as a placeholder for either "Alexa" or "Hey Google".

Privacy reasons as well. Every ounce of what you say after "Alexa" or "OK, Google" is submitted to Amazon/Google.

I don't see them changing this ever, so having a recognition phrase is perfectly fine by me.

Performance may be a small part of it, but it's more about branding. The trigger word is the brand, the functionality is the product.