Yes, it's a daft rule. But wherever you set the limit it's going to be a daft rule...
The top-spec 450kg microlights have constant speed props, retractable undercarriage and so on, and are in reality 500-550kg machines. If the weight limit went up to 500kg, they'd have bigger fuel tanks, full panels, autopilots and bigger fuel tanks, and there'd be no way you could get 2 people into them while respecting the 500kg limit!
Where the rules are daft (in this country at least), are that you can buy a ready built microlight as long as it is 450kg or less. But if you buy exactly the same aircraft as a kit, you can certify it as an aeroplane and have a much higher MTOW.
Personally, I think the factory would do a rather better building job than me.