Not necessarily. If the interest rates are reduced because the economy improved, then the growth could stay similar.

There's no guarantee at all. In a really bad year (2008?) you might get -50% or worse. Then you might get some really nice returns in the years after (or not!). If you average over several decades, historically, the returns have been around 7 or 8 per cent per year, but the standard deviation is enormous. Just look at a long-term chart of e.g. the S&P 500 at https://finance.yahoo.com/chart/%5EGSPC - click "Max" and "Settings" -> "Logarithmic" (you'll want a logarithmic axis so that equal percent changes are equal distance on the plot). You'll see that on average it went up over the decades, but between June '07 and February '09, it lost over 50%, and tripled since then.

I encourage you to read up on this, but someone else with more knowledge should recommend some books.

Banks operate on a completely different model so it's not a useful comparison, but the answer is that they are borrowing at ~0% and lending 2%+ so they view the world differently

banks are heavily regulated and cannot just put deposits (the money you put into a bank constitute a loan to the bank) into stocks. They have to invest more conservatively, e.g give money to solid companies or hold low risk bonds. See Basel-rules to learn more about risk weighted assets (RWA): https://en.wikipedia.org/wiki/Basel_III

>if I can just go on and get 7% on average?

I didn't mention any numbers. If economic growth is low, and inflation is non-existent, you will not approach 7%.

>Why are banks struggling to get even 2% return on their investment

Where do you get that information?

risk --> reward.