Because the law looks at intent. If you do nothing other than legally change your sex right before applying for a grant they'll say it's fraud and a jury is likely to convict. Remember all this bullshit only works one way. They know it's fake. They're not going to let you get away with playing the system because the whole point of the system they created is to put you at a disadvantage. You can only play the system when arguing on twitter or dealing with some gender studies true believer.