Amy - Your logic may work for instance when an employer wants to check your background, but the I-9 is different because it's a government form designed for their purpose of identifying legal vs. illegal workers. On the now lame-duck form the guidance regarding the "Other Names Used" field was rather vague* and the space had not been adjusted to fit a longer full name (so a TG might have been able to pull off putting something like just their old initials or last name if different). On the new form the guidance is more precise, one of the form versions is a "smart" form which among other things won't accept an incomplete form (they say to put "N/A" in the field if you've not used other last names), and the penalties on the employer for form completion errors have been heightened. *This also meant there was ambiguity about whether or not nicknames, truncated hyphenated names, etc. had to be included.
I think why USCIS came up with the compromise they did was:
1. Legal changes to the last name are very common (e.g. marriage/divorce) while official changes to the first name are less frequent (and when done are more likely to have documentation changed in a timely matter than for example how some women when marrying are slow on switching their documents).
2. There is more likely to be an issue of the former name itself giving away why the change was made with first name changes than last name ones. Obviously there are our cases as well as cases like where their parents went "crazy" with naming their children (which may have also factored into USCIS's decision, citing transgender people as well as other more general circumstances where discrimination is possible for their decision). While the issue is not nonexistent with last name changes (e.g. immigrants "Americanizing" their names, religious conversions, etc.), the bulk of last name changes are due to family reasons (which other than marriage/divorce include adoption, paternity issues, family associations, etc.) where although the person may prefer not to disclose it for personal reasons just giving out said former last name would not indicate the reason for the change.
3. When someone tries to change their name for illegal or fraudulent reasons it's less common for them to change their first name alone (more often they do the full name or just their last name).