Because we would look desperate to sign a striker and teams could use that against us. Lyon aren't selling Lacazatte for 50m to Arsenal, but they'd probably take another 10 or 20m I'm sure and would probably raise the fee to that if we came knocking.
What exactly makes Morata "unproven" and Lacazatte "proven". Last time I checked, Morata has played in Spain and Italy, two MUCH superior leagues (especially Spain), had key goals and performances in the Champions League and has played (and scored) at a major International tournament. Lacazatte has done none of those things.
On top of that the qualities Morata brings seem more suited to a Mourinho team and our system and also importantly, he has a winning big club mentality. Always played with winners. World Cup winners, Champions League winners, League winners and has played an important role in the winning of all those wins apart from the World Cup.