Lack of research opportunities in Israel
Are there any reasons for Israel seemingly not being (a) involved much in international research collaborations and (b) interested in foreign students/researchers?

Your assumption (a) is completely incorrect — it’s hard for me to think of a statement that’s further than the truth.
Assumption (b) is correct in the context of foreign students, and this has a simple explanation. It’s mostly a language issue: PhD programs in Israel are offered in Hebrew, and it’s extremely rare for foreign students looking for a place to do their PhD to be willing to invest several years to learn the language at the level they’d need to know it to be successful in doing a PhD. The ones who would entertain that possibility are small enough in numbers, and highly motivated enough, that there’s no need for graduate programs to do special outreach targeted at them. Any postings for PhD programs or positions will be in Hebrew, and if you are not a Hebrew speaker you will not be able to find and read them.
There are a few exceptions to this. The Weizmann Institute of Science has English as its official language and does attract a reasonable number of foreign students. Tel Aviv University has an English speaking medical school program and, I vaguely recall, a business master’s program. There might be other isolated programs for which the language barrier wouldn’t be an issue.