I don’t even understand why devs never thought about this!
To build interesting team, you need to see in advance all available options to make best synergy possible within your team.
It’s not even that hard to be done, starting arena UI could look eg. like this:
Clicking on card would bring you card details and give you option to Select that card (what is marked with green border). Clicking Start! button without selecting 4 troops (1 from each section), would give you “You have to select one troop from each section!” error.
This is just an example, there are many ways to implement team selection in reasonable way!