This is one of the harder things to explain, but let me try: Overwatch and Apex Legends aren't the same genre. This may seem like it, what with them both being FPS games with characters that have unique abilities, but they're not; and I'm not even talking about BR vs objective based team deathmatch (although that of course figures in too). I'm talking about the fact that Overwatch is primarily an ability based game, and Apex is primarily a gun based game. Sure Overwatch has guns, and you've got to learn how to be good at shooting them; Soldier 76 and Widowmaker have very different guns that require different skills to master, for instance. But importantly, these guns exist exclusively in the context of the rest of the kit. You couldn't just put Soldier's gun on Zarya and call it a day; you couldn't even put it on Tracer, another character who's in the same class as Soldier.
In Apex, you can learn to be good at controlling the Flatline, and now you're good at controlling the Flatline with any character. There are subtle differences, such as Rampart having a shorter reload time on LMGs, or large characters having slightly different sightlines due to elevated eye position compared to small characters (Caustic can shoot over obstacles that Wraith can't shoot over, but Wraith can hide behind cover where Caustic's head would poke out over it), but mostly you have the same skillchecks on guns no matter what character you play; this also means you can learn to play around the lethality of other characters based on what guns they have; you get a feeling for how long it takes someone to burn you down with an R-99 compared to a Spitfire.
Neither one of these models is inherently better or worse; I would argue that the gun based model is slightly better for a one life mode (like I said, the BR thing does figure in) because you can't be surprise-killed by a character's abilities; it's the guns that do the killing, and you should already be used to them. Even when we introduce a new gun, there's a high chance you'll run into it regardless of the enemy characters you run into, so you get ample opportunity to practice dying to the Volt (I know I have). I know ability based BRs *can* work (I happen to think Realm Royale is a really good game; I just never get enough time to really get into it), but they probably mean you can't keep releasing new characters (which they don't) and you have to kind of overcommunicate everything that happens (which is why fantasy/magic is such a good fit for Realm Royale; they get sparkly magic VFX that they can use to REALLY overcommunicate what just happened).
So looping around to your original point, why characters in Apex are generally weak on release (and if you couldn't tell, as someone working on the Legend meta, I've been thinking about this rather a lot recently ;P): my theory is that being a gun game precludes us from using certain power levels for abilities. For instance, do I think Horizon will be strong on release? I sure hope so! We made some last minute adjustments to give her a little extra power and some of us (myself included) are quite worried we overshot it, but we had the same concern with Rampart and Loba and release Revenant, so who knows. But what I know for a fact is that abilities have an upper boundary in Apex in regards to the power they can express. For instance, we couldn't have a Junkrat type detonation pack that does meaningful damage on demand and has the potential to boop you off the map (boop is a term of art in game design, don't worry about it). So we kind of have to try and find that narrow space where an ability is useful enough that you want to bring the character while also making it so guns are still the primary deciders of combat outcomes. When we honestly can't tell from our playtesting how much of an impact it'll make, we do err on the side of caution.
It's an interesting philosophical debate: do we do more damage to the game by releasing underwhelming characters that then have to be buffed up over the next 2-3 patches, or do we do more damage to it by releasing broken strong characters that have to be nerfed into the ground, possibly with a hotfix? Conventional wisdom says the later does more damage to the game: it opens us up to accusations of "you're just making the character strong so we have to buy it on release and then you nerf it" (League of Legends flashbacks intensify) and the excitement of having a new character can turn sour real quick if every time you meet them they kill you in a way that feels bullsh*t (I'm sure that's not going to come back to haunt me). That said, pretty much every character we released post launch was either weak on release or underappreciated by the community at large (I'm thinking Wattson here; she was always strong, but mostly played in competitive only.)
TLDR: we're not trying to ship weak Legends, but it's really hard to make Legends strong in a way we can be confident won't ruin the game for everyone for a week or two. We'll continue buffing Legends that release weak.
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