Corporate tax is always only paid on profit and is usually a minor part of the tax draw for the government from corporations of all sizes.
The vast majority of taxes paid in developed nations are employee taxes and whatever national+local sales taxes and health/pension equivalent taxes are (indirectly) levied (usually 60-80% of national income). Asset taxes are a bit different.
It's true even in the bootstrapped company case:
If you earn say $100k and keep $50k after all the employee indirect/direct taxes. Now imagine you spend $40k of that $50k in savings, setting up a business. You spend $30k on another employee, paying $15k of employer and employee taxes, and spend the other $10k on a company to do marketing (who will spend $5k of that on employees and pay $2.5k of tax), and you earn less than $40k in income, by the end of year 1 you have:
1) A loss-making startup which nonetheless is further along then nothing
2) Out of $100k of your original value, $67.5k has already reached the government within 12 months
3) Your time doing the tech side was not compensated but could not (for obvious anti-fraud reasons) be counted as a loss and as you have noted, you don't pay tax when you make a loss, and you don't get any kind of negative rebate (except certain sales tax regimes or schemes).
If you are in the US, the above is currently much worse due to the insane way R&D Software spend needs to be spread immediately as a tax burden.
So it's really not fair to say a new startup isn't paying taxes. They almost always are. There are very few companies or startups that pay less than 50% of their income to staff, and almost all of those are the unicorns or exceptional monopoly/class leaders. Startups, and founders tend to disproportionately give more of their income and are essentially to that extent re-taxed.
Even though you saved the money in order to start a startup, and paid your due employee taxes, you then have to pay employee taxes to use it, etc.