Or tourists across the board are also spending less than they used to due to the economic situation in their home countries. Tourism like every other industry isn't a smooth guaranteed upward trend of increased revenue, in many ways the opposite since it's discretionary spend.
What are you suggesting :
1) Stop admitting tourists when you hit a certain arbitrary number that somebody thinks is enough
2) Only let people in if they promise to spend a certain amount per day
HIstory tells us Thailand is very proactive at adjusting its visa policy when economic necessity dictates. Is it the least bit surprising given what COVID reaped tourism wise and the general economic situation here at the moment, which isn't great, they are trying to get tourism numbers back to previous levels.
What is the alternative employment wise when you are trying to stimulate the economy ? Seems to me I've read quite a lot lately about factories closing in traditional big employment industries like car manufacturing, laying off several hundred workers at a time on the one hand, whereas the MOU's signed for the new tech they are trying desperately to attract will take time to develop.