The reason it seems more off-topic/spam posts than the per-site Meta's is because it is the only meta site without a reputation barrier (despite the listed "participate in meta" privilege [see http://meta.stackexchange.com/questions/254054/remove-meta-privilege-for-mse-only for the reasoning]).
Anyone can ask a question here and it doesn't even require an existing account on another SE site. Yes, this means it can lead to some spam and off-topic questions, but there are a number of benefits:
- It gives a voice to low rep users who cannot participate in their own site's child meta. While the question will probably get closed since it only pertains to a specific site, they usually will get feedback via comments and answers before that happens.
- Bug reports/feedback from users without accounts
As for the Spam/Off-topic questions - how long does it usually last? There is more than enough activity on the site to get spam nuked within minutes, and usually the off-topic questions get closed pretty quickly, and deleted not to long after that.
Echoing what I said in my comment, if you turn MSE into a "High Rep Members Only" club, you only further isolate low reputation users from contributing to improving the Stack Exchange sites.