Does seem a bit mad to not have direct trains between what is arguably South London's primary rail hub and South London's primary seaside destination purely due to timetabling decisions.
It's worth bearing in mind that whilst for some of us a simple change isn't problematic; there are many people (those with buggies, with heavy luggage, with mobility issues, general anxiety about travel, those non experts/enthusiasts who find rail travel stressful) for whom an extra change will simply put them off travelling at all, or push them to driving. I know of quite a few people who will choose their travel mode, or their day trip destination, based on whether or not there is a direct train. It won't be obvious to them that there is an 'easy cross-platform interchange at Haywards Heath'. They'll simply see that a journey from SWR-network station to Brighton now requires two changes and they'll think 'that's too complicated/stressful, I'll drive instead', or 'I'll go for my seaside trip to Bournemouth instead'.
Traffic figures suggest that's not true. Clapham off peak already has 6tph direct to Gatwick (2x Littlehampton, 2 x Eastbourne, 2 x Arun Valley (Portsmouth/ Southampton/Bognor). There is absolutely no need for Gatwick Express to call at Clapham.
You're clearly not a regular user of Clapham Junction to Gatwick Airport trains then. They're often (but not always) packed out with barely any standing room left when leaving Clapham Junction; not ideal for people with lots of luggage and/or children. A 25% or 33% increase in capacity by stopping an extra 1 or 2tph respectively is very-much needed.