Let's start with the easy parts of how you're wrong. Go pick a fight with regular bad guys and drive away. Note the XP you get.
Then, go murderer however many cops you likerer, drive away, and then take note of how much experience you're awarded for getting the whole cast of NCPD Blue to shake hands with Jesus. The digits'll be thin I assure you.
As to what corpos hold sacrosanct? What system corpos adhere to? Well that's obvious that it pains me to have to explain it choom... the system of entrenched power.
Corpos are, almost to a man, adherents to the church of *self preservation*. They have something to lose and they don't want to lose it. Thus the benefit of corporations: *stability*. What does an individual already on top want more than almost everything else? To stay there. Something something first rule of power.
Notice how none of the corporations are defined by what they believe (instead defined by what they do) while *all* the gangs are defined by what they believe? The quasi catholic Valentinos, the patriotic sixth street, the hedonistic/yakuza oriented tyger claws, and so forth. These organizations are defined by what they believe. Corporations are defined by what they do.
This might be hard to parse... but what does "Arasaka" believe? If you dig deep the original purpose was Japanese World Domination... but that dream is long since dead for all intents and purposes. Militech is a quasi branch of the Federal Government yet you'll notice nothing especially patriotic about them.
Consider a mission where you break in, eliminate guards, assassinate someone, and recover data... you engage in the *exact same functions*... but if you do it with the backing of a fixer to be paid off by the valentinos, or you do it at the behest of a corpo depositing eddies into your account via militech... the motivations for *why* such actions need to take place, who you're eliminating, the amount you'll get paid and more will differ dramatically.
For example, if you're a thug in the employ of the valentinos and get caught... the valentinos don't care. Conversely if you're an operator for Militech and you get caught *that's a PR nightmare* that needs to be dealt with.
If you're high up in rankings at militech, a known quantity that can accomplish the impossible you'll rake in tremendous cash and have access to the very best militech has to offer. Maelstrom would probably pay money to the first gonk who managed to complete the job, but would more likely than not just zero them too.
Are you seeing how the systems are different? The Streets can afford fuck ups and failures, corpos cannot.