It's because shit sides like my Dragon's can only attract plodders and players after a last pay cheque and we have to pay overs even to get them to fill a 30 man roster while the top sides are able to pay unders with a carrot that you can play semis and win a comp and life afterwards.
If there was no requirement to spend 95% of your cap, you could pay a plodder say 300k and not 400k which then leaves room to pay for better quality players when they come on the market.
As someone noted above, if all clubs are spending the same cap and barring any significant TPAs or back door deals, why is there such a huge difference between the squads of the top 2 or 3 teams and the bottom 2 or 3? Can't all be down to incompetence of club administrators and recruiters.