Because as far as I can tell, the technical data leads to that assumption.
So the vanilla Securitron Mk. 2s (which can be treated as the "full" Securitron, cause really the only improvement over the Mk 1 is a software patch) clearly beat out the Sentry Bot in firepower in NV, containing everything the Sentry Bot has in firepower, which at best is a Gatling Laser and a Missile Launcher, however the Securitron has even more, with a second missile launcher, a grenade launcher and a submachine gun as a cherry on top.
What is something that's more nuanced and debatable though, is the durability of a Sentry Bot or a Securitron. In this case, the raw numbers are kinda more a suggestion than outright fact. However oddly enough, it still points in favor of the Securitron.
The Mk 2 Securitron has two different versions in NV, one in the Battle of Hoover Dam, and one used everywhere else. The Hoover Dam variant has 450 health and 30 DT, while the other has 125 and 20 respectively. The Sentry Bot meanwhile has 300 health and 18 DT.
Now there is a bit of a debate here as to which is supposed to represent the real Securitron, however luckily we don't have to go down that rabbit hole. For you see Old World Blues has improved versions of both robots. These improved ones both max out at 675 health, suggesting, in combination with the two different vanilla numbers, that the Sentry Bot and Securitrons have around the same amount of durability.
But here is the kicker: The Securitrons in Old World Blues, being "broken", has no DT. But even the improved versions of the Sentry Bots have the same DT, of 18.
So, using this information, we can infer that the Securitron and Sentry Bot have roughly equal durability, but the Securitron does have a slight edge in armor strength.
This is a pretty crazy thought, as while later editions of the Sentry Bot do have more weapons with their two grenade launchers, it's still mostly on par with the Securitron at best. And since the health and especially armor mechanics of those later games are different, we will just have to make do with this one health comparison.
What do you think? Do you believe I'm right?