When the Jedi set out to Geonosis, it wasn't explicitly a rescue mission (though, they likely assumed Obi-Wan had been injured/captured). They were after Dooku, the Trade Federation and the Droid Army being assembled.
Obi-Wan has witnessed the factory, Dooku and the Trade Federation members and relays this information to the Jedi along with his conclusions.
From the script
OBI-WAN: (V.O.) I have tracked the bounty hunter Jango Fett
to the droid foundries on Geonosis.
INTERIOR: JEDI COUNCIL ROOM - DAY
The members of the Jedi Council stand around a hologram of
OBI-WAN.
OBI-WAN: (V.O.) The Trade Federation is to take taking
delivery of a droid army here and it is clear that Viceroy
Gunray...
INTERIOR: COCKPIT, NABOO STARSHIP - DAY
ANAKIN and PADMÉ continue to listen.
OBI-WAN: (V.O.) ...is behind the assassination attempts on
Senator Amidala.
INTERIOR: JEDI COUNCIL ROOM - DAY
The Council members continue to listen to OBI-WAN.
OBI-WAN: (V.O.) The Commerce Guilds and Corporate Alliance
have both pledged their armies to Count Dooku and are
forming an... Wait!... Wait!!
INTERIOR: COCKPIT, NABOO STARSHIP - DAY
ANAKIN and PADMÉ watch as OBI-WAN is attacked by Droidekas.
The hologram cuts off. ANAKIN jumps up, agitated.
INTERIOR: JEDI COUNCIL ROOM - DAY
The Council members also see the attack on OBI-WAN. YODA
looks to MACE WINDU.
YODA: More happening on Geonosis, I feel, than has been
revealed.
MACE WINDU: I agree.
INTERIOR: COCKPIT, NABOO STARSHIP - SUNSET
ANAKIN and PADMÉ watch a hologram of MACE WINDU.
MACE WINDU: (V.O.) Anakin, We will deal with Count Dooku.
The most important thing for you is to stay where you are.
Protect the Senator at all costs. That is your first
priority.
But the Jedi are actually being played by Palpatine.
Palpatine is putting the final pieces of his plan to start a war into place. He manipulates the situation into sending the Jedi into a battle where he likely hopes at least some of them die.
INTERIOR: CORUSCANT, CHANCELLOR'S OFFICE - DAY
BAIL ORGANA: The Commerce Guilds are preparing for war...
there can be no doubt of that.
PALPATINE: Count Dooku must have made a treaty with them.
BAIL ORGANA: We must stop them before they're ready.
JAR JAR: Exsueeze me, yousa honorable Supreme Chancellor,
Sir. Maybe dissen Jedi stoppen the rebel army.
PALPATINE: Master Yoda, how many Jedi are available to go
to Geonosis?
YODA: Throughout the galaxy, thousands of Jedi there are.
To send on a special mission, only two hundred are
available.
BAIL ORGANA: With all due respect for the Jedi Order, that
doesn't sound like enough.
YODA: Through negotiation the Jedi maintains peace. To
start a war, we do not intend.
ASK AAK: The debate is over! Now we need that clone army...
BAIL ORGANA: Unfortunately, the debate is not over. The
Senate will never approve the use of the clones before the
separatists attack.
MAS AMEDDA: This is a crisis! The Senate must vote the
Chancellor emergency powers! He could then approve the use
of the clones.
PALPATINE: But what Senator would have the courage to
propose such a radical amendment?
MAS AMEDDA: If only Senator Amidala were here.
JAR JAR steps forward from the back of the group.
JAR JAR: Mesa mosto Supreme Chancellor... Mesa gusto
pallos. Mesa proud to proposing the motion to give yousa
Honor emergency powers.
So, why are so many Jedi risked? Multiple reasons:
- From the Jedi's perspective, to stop Dooku and the newly formed alliance and their army
- From Palpatine's ulterior motives, to begin the Clone War