“I think” can also be “saya pikir” without “ber-“ so I think this should have more context to avoid confusion.
You can say "saya pikir" in colloquial/informal speech.
Absolutely no problem with that, because the affixes are usually dropped in colloquial speech.
However, the issue here is that the root word "pikir" is a noun and not a verb.
That's why the "ber-" prefix is used here (to make a verb out of a noun).