Probably because we didn't buy one last year, the last two we bought were a free transfer who's been forever injured and a 19 year old up and coming player. Oh, and because our top scorer is a winger/wide forward with 13 goals. Behind:
Hazard 14
Ali 16
Costa 17
Ibrahimovic 17
Sanchez 19
Kane 20
Lukaku 24
You could say from that lot, only two are "carrying" their teams, in Lukaku and Sanchez. The top two teams in the Premiership have two players combining to give a total of 31 goals (Chelsea) and 36 goals (Spurs - Son also has 12). Our top two goal scorers give us 24 goals, neither of whom are actual strikers.
And so it's proved, what? United don't rate Rooney these days, clearly. Rashford has hardly played and Ibra has one season left. So if they buy one then it'll be because they need one.
If we buy one, it'll be because we need to keep improving and because there's room for a more reliable source of goals as well as the quality we already have. Nobody said Firmino "can't be trusted/isn't good enough/should be replaced", as I've reiterated throughout, yet people keep seeing the suggestion that we need a striker meaning as much. I'm not really sure what your argument is here. If we buy a genuine goalscorer then it'll be because we need one and they'll be in the team, scoring goals sooner rather than later.
In the top 25 scorers in the league, Spurs have 4 players, combining to add a total of 55 goals. We have 3 players, adding 33 goals. Our overall record is better, but spreading the goals across the team isn't always reliable. We can't guarantee regular goals from Lallana, Wijnaldum, Henderson, Can, or the defenders, other than penalties from Milner. Our front three add the bulk, but not in the lofty regions of a Costa, Kane or Ibra, they're not far off, but it's hardly a given. Coutinho's goal return is fairly steady and doesn't improve season on season, Mane is a real threat, Firmino is a false 9 who will weigh in and looks like he might get 15 at best. Though that's without accounting for the fact that those above him will also continue to score until the end of the season, running into 20 something goals.
I'd be really surprised if we don't add a quality player to the front line, and not because it's the done thing to buy one most seasons - because we need one to push on further. That's not a swipe at Firmino or any of the others, they will continue to be a part of a wonderful attack, just hopefully one that can be even better. I'm not sure why that's so unfathomable, we can and should always strive to get better.