The reasons could vary depending on which team will win the match, but I see it differently, as there is a possibility of a draw, reminding both teams that they will be cautious enough to avoid dropping points.
One striking difference between the 2 teams is that Liverpool have conceded 4 goals, while Arsenal haven't conceded in their previous 2 matches. Both teams are performing equally well in attack, although Saka was absent from this match, but Arsenal still have reliable substitutes.

According to the bookmakers, Liverpool are the favorites, most likely because they are playing at home. In the previous match, Liverpool hosted Bournemouth and conceded 2 goals. Therefore, a draw is still possible, but the best betting option is most likely BTTS.
A draw is very possible because Liverpool and Arsenal don't have a significant difference in strength. However, betting on a draw is very risky, and I agree with you that BTTS is the right bet in this match.
Liverpool have conceded in their last two matches, which is the opposite of Arsenal's, but I think things will be different in this match. After all, Liverpool and Arsenal are two top Premier League teams vying for the title, so this is a prestigious match and a testament to both teams.