Why are superhero animated movies not made with the same level of graphics as pre-rendered videogame cutscenes?

There are multiple reasons here.

Money

Simply put, cartoony graphics are considerably cheaper to develop than the high realism 3D cinematics.

Animated cartoons don't have the biggest audience (compared to live action movies). The smaller the profits, the smaller the development budget.

Without having seen the numbers (I can't find any reasonably trustworthy source), I will gladly bet that the budget for the Arkham games was considerably larger than for the movies you linked.

Stylistic choices

Some animated 3D movies opt for a cartoony vibe that avoids excessive graphical realism. This can be done for many reasons. It's very prevalent in (but by no means limited to) children's movies. Let's use Shrek, Inside Out and Bee Movie as example, but I'm sure you know the list is quite extensive.

A great example here of stylistic choice are the Animatrix movies. This is a collection of short films related to the Matrix universe, with varying graphical styles, from realism to anime. Each movie stands on its own, and there's no reason why they should all have used the exact same graphics, let alone all needing to try for as much graphical realism as possible (which is what your question is trying to assert).

Nostalgia

This is arguably a subset of the stylistic choice category.

Movies like the linked Batman movies very much herald back to the age of the comic book. The graphics retain that thematic connection. There's also the connection to older animated series (e.g. Batman: The Animated Series) on top of the comic book connection.

The comparison is easily made here. The Arkham Origins trailer seems much less related to Batman comic book ancestry than both Hush and Gothan By Gaslight do.

They actually do, sometimes

For their time of release, some movies and shows do heavily lean on realistic graphics:

• Final Fantasy: The Spirits Within (2001) had equivalent graphics to the Final Fantasy game cutscenes released around the same time.
• Beowulf (2007) was a frankly astonishing marvel in terms of facial modeling of actors (most notably Anthony Hopkins)
• Harlock: Space Pirate (2013). Maybe not the greatest movie by itself, but a very interesting realistic 3D take on an anime.
• The Animatrix (2003) - at least some of the short films, as discussed above.
• Love, Death & Robots (2019) - at least some of the episodes, similar to the Animatrix.
• The Lion King (2019). My opinion on the remake aside, you cannot deny its astonishing graphical quality.

This list is merely from recollection, i.e. movies that astonished me with their graphical fidelity (relative to the year of release, of course!). The list is much longer.

In addition to Flater's answer, I feel it's worth emphasizing that Batman: Hush and Batman: Gotham by Gaslight are both direct-to-video productions - they are not intended for theatrical release, but are instead sold as DVDs or Blu-Rays. Direct-to-video productions don't make anywhere near as much money as theatrical releases: as of 4 January 2020, Gotham by Gaslight has made $4.6 million, and Hush has made $3.6 million.

Because of this, direct-to-video animations have to be made very cheaply in order to turn a profit. This means using traditional 2D animation, as cutting-edge 3D animation like in the Arkham games is extremely expensive, and anything less than cutting-edge will just look terrible.

2D animation is also far quicker to produce, and as a result, multiple productions can be released in a single year. DC have released at least 3 direct-to-video animations every year since 2009, and at least 5 every year since 2015. I don't know of any 3D animation studios that have that kind of production rate.