If capital gains are taxed, whether it’s realized or not the liability exist. If the tax is flat (not progressive), the same amount of money will be collected, right?
(Outside of shenanigans like cost basis reset on inheritance which don’t make any logical sense in term of fairness)
Btw the gouvernement is not an abstract thing, we’re talking about democracies, where people vote (often to protect their social benefits that need to be funded). It’s usually the people (directly or indirectly) that decide to raise taxes.