No player is a guarantee, look up the stud WRs in the NFL and their draft selection spot, you can find them all over the draft.
You can't say the same for QBs
You're right, no player is....but if you look back at the last 20 drafts, a WR taken top 5 had a better success rate.
Ja'Mar Chase
Corey Davis
Amari Cooper
Sammy Watkins
AJ Green
Calvin Johnson
Braylon Edwards
Larry Fitzgerald
Charles Rogers
Andre Johnson
The worst guy was Rogers and he ended up having a issue with prescription drugs and done after 3 seasons.
Now here's a list of QB's taken Top 5 over the same time:
Bryce Young
C.J. Stroud
Anthony Richardson
Trevor Lawrence
Zach Wilson
Trey Lance
Joe Burrow
Tua Tagovailoa
Kyler Murray
Baker Mayfield
Sam Darnold
Jared Goff
Carson Wentz
Jameis Winston
Marcus Mariota
Blake Bortles
Andrew Luck
Cam Newton
Sam Bradford
Matthew Stafford
Mark Sanchez
Matt Ryan
JaMarcus Russell
Vince Young
Alex Smith
Eli Manning
Phillip Rivers
Carson Palmer
The guys drafted last year are to new to evaluate in this list. Russell, Sanchez, Bradford, Bortles, Wentz & Darnold are bust. Lance & Wilson are flirting with being bust as well. Vince Young had a good rookie season and then fell off. Winston & Mariota have just hung around in back-up roles waiting for someone to get hurt.
For me its not very often that you have a WR whose in the discussion for a Top 5 pick. Its even more rare for a WR to have the talent to go Top 3 in the draft. In the last 20 years the WR's I bolded were Top 3 picks. 2 of them are HOF and 1 (Fitz) will be a HOF. Edwards carved out a decent career while Rogers busted because he got addicted to pain medications