Broker(s) rarely suggesting smaller lenders vs big banks?

I recently spoke to a couple (4) mortgage brokers to understand my borrowing capacity and also the best rates I can get for an investment property loan. Almost all of them mentioned that they would look for the best rates across a range of different lenders both big and small but all 4 of them came back with one of the big 4 banks as their recommendation and an interest rate that was around ~6%.

Coincidentally, I walked past the "Aussie" lenders the other day and noticed in their window an interest rate that was considerably lower (0.4-0.5%) than what all my mortgage brokers had mentioned. Zuckerberg must also have been listening because later that night I saw an ad for Bank of China with a lower interest rate, clicked into it and also saw an interest rate that was lower than what my brokers had recommended too...

Not sure if I might be overlooking something, but do the mortgage brokers have an extra incentive to bring customers to the big 4 banks (even if the interest rate isnt the best?), or did they genuinely look across all the various lenders, big and small, and it just happens to be that the big 4 banks are the best for what I need?

Thanks so much!!