Because sex is a protected characteristic in the Equality Act 2010 and the rights of women and girls depend on it.
Because 'gender', which is defined nowhere in law, has too many different meanings, some of them heavily contested, and the insistence on gender in place of sex is what has eroded our rights.
Sex is the Jif and the Marathon in your analogies by the way, gender is the Cif and the Snickers. Gender is the johnny-come-lately word that arrives with a ton of baggage and a dodgy agenda. This is not natural language evolution this is forced change (with menaces).
When it suits the agenda, gender is just a polite euphemism for sex.
When it suits the agenda, gender is a mysterious internal subjective quality which, it is claimed, everyone has and which is completely different from sex.
The confusing fudge that results from mixing up these different meanings is why men have succeeded in claiming to be women and claiming protection under the equality act as if they were women.
To confuse matters further, gender also has another meaning - sex roles / sex role stereotypes / sex role indoctrination. All the stuff we do to children, and to each other as adults to enforce conformity of masculine and feminine behaviour. All the stuff feminists have been battling against since forever.
To confuse matters even further, a lot of men who claim they have a 'gender identity' in the 'mysterious internal subjective quality' sense of the word, are actually just getting off sexually on the humiliation of pretending to be a woman because they understand the 'sex role' meaning of gender very well indeed.
Obviously this is a problem for women when these men demand to be treated as women under the law, because of their 'gender'.
Gender can get in the bin.