If I go and temp in a role I can choose not to turn up at next week after doing this week because it was convenient, and I'm doing the basic, vanilla, entry level work & putting my pen down at the end of hours and going home. I won't get paid the same rate as I do for my 20 years professional experience, no.
If I want paying for the full range of experience I have, then I need to look for a role (temporary or permanent) that matches the skills I have to offer. Not something that needs a bum on a seat meeting the basic ability to deliver entry level skills.
If you want paying at scale, either negotiate with the Agency before signing, do supply direct to a school rather than via an Agency, or find a role for more hours on a permanent basis.
If you want to bring your 21 years experience in YR-Y6 to bear on flitting in to cover teaching Y1 some phonics & playtime for a couple of days every now & again, when a newly qualified without a permanent role yet could do it for £21.54/hour. Do I think it would be a good use of taxpayers money paying you at scale?
No.
If you don't like that answer, go negotiate yourself a different contract somewhere else.