This is nonsense for the vast majority of the developed world. Governments aren't elected by the world, they are elected by the citizenry of their respective nations - a citizenry that absent of immigration would be on a long term decline in most of the developed world , and yet those governments largely continue to push for immigration not so they can work harder to drive economic growth to keep all these new citizens happy with a 'pizza slice' that stays the same size, as you put it, but precisely because it's easier to drive growth as you expand both demand and supply of goods and labor through population growth.
The root cause of growth-above-all is simple - when you have more, more stuff, income, experiences, you get used to it very quickly and your level of happiness quickly adjusts back to what it used to be. This is known as the
hedonic treadmill. Once that high wears off, you start to chase it again by acquiring yet more - a job that pays more, a nicer car, a bigger house.
Once you understand this aspect of human nature, everything else becomes obvious. Citizens might have a lot of secondary concerns like values and 'does this candidate look like a good person', but ultimately who most people vote for is who they think (erroneously or not) could make their material circumstances better tomorrow. So, faced with a citizenry that wants to see their lives improved, what can they do? They could redistribute the pie - tax the 1% more and transfer wealth to the less wealthy and in doing so make 99% of the population more well off, but then they very quickly confront the reality of the global economy in that wealth has become extremely mobile. The wealthy would just move their wealth to some other country. In some countries the wealthy have also turned the spinning of redistributive taxes into an art - let them tax me more today and tomorrow it will be you, if they tax me more I will have to pay you less, etc.
It turns out its a lot easier to just grow the overall pie, even if as a result of that the average joe sees his material circumstances improve by negligible amounts (or even decrease in real terms), and most of the benefit accrues to the ultra-wealthy. This political reality is why most governments continue to focus almost exclusively on overall GDP (which ignores distribution completely) and GDP per capita (which is heavily skewed by the highest income households). This is why so many developed countries immigration policies overwhelmingly favor the wealthy - who on a per-capita basis have biggest impact on GDP (by driving up demand for luxury goods) and in the short-term at least, do not drive down absolute wages for the common man (of course by driving up prices means those salaries end up being worth less in real terms, but lets not talk about that).
This is an oversimplication of course, there are many structural issues that mean stagnation or negative growth would wreak havoc on an economy, but you can usually understand those issues through the lens of man's basic desire to have more. For instance - debt carries an interest because the lenders want their capital to make more money (not just because of the risk of default *requires it,* as economics 101 would tell you), and for it to make financial sense to take a loan, a business or a homeowner would have to grow its profits or see the value of their house appreciate by at least as much as the interest on the debt they've borrowed. Taken in totality, you have a vast swath of society who need profits and prices to rise faster than the size of their debt, and incidentally you also end up with a system that very quickly goes wrong once growth stalls.