Why do astronomers not use past tense when discussing observations of the universe
You're being a pedant and are not necessarily correct!

The present tense could be clearly defined as what is experienced at the time of observation. To write that it had a black hole 70 million years ago would (pedantically) only be correct at one instant of time, not correct when the reader read the sentence and even ignoring that pedantry, not necessarily true for gravitationally lensed light where the light has not travelled along a straight path!