Because there are no Prerequisites to take the CFI as listed in 61.39 except for correcting the knowledge test which you can just sign the test form. AND 61.187 pretty clearly states you need the Flight Proficency/Practical.
The 61.39 endorsement is needed for all checkrides. The old Part 61 FAQ's said that while there is no minimum amount of training time needed for the CFI, it must be non-zero. I've copied below what the pilot examiner's handbook says. The parts in red are the 61.39 endorsements.
B. Logbook Endorsement. An applicant for a Flight Instructor—Airplane or Flight Instructor—
Glider rating must present a logbook endorsement from an authorized flight instructor who indicates the applicant is competent and proficient in stall
awareness, spin entry, spin, and spin recovery procedures in an aircraft of the appropriate category.
(1) Except as provided in § 61.185(b), an applicant for a flight instructor certificate must have logged training and received a logbook endorsement
on the fundamentals of instruction listed in § 61.185(a)(1).
(2) The applicant’s logbook or training record must contain an endorsement from an authorized instructor who certifies the applicant has received and logged some training within the 60 days preceding the date of the application in preparation for the practical test.
(3) The applicant’s logbook or training record must contain an endorsement from an authorized instructor who certifies the applicant is prepared to
pass the practical test. In addition, the applicant must have a signed FAA Form 8710-1, Airman Certificate and/or Rating Application, with the authorized instructor’s signature.
(4) The applicant’s logbook or training record must contain an endorsement from an authorized instructor that states the applicant has demonstrated satisfactory knowledge of the subject areas in which the applicant was deficient on the airman knowledge test.
(5) An applicant for a flight instructor certificate must have logged training and received a logbook endorsement from an authorized instructor on
the training required by § 61.187(b) that is appropriate for the flight instructor rating sought.
(6) An applicant for a Flight Instructor—Airplane or Flight Instructor—Glider rating must present a logbook endorsement from an authorized instructor indicating that the applicant is competent and possesses instructional proficiency in stall awareness, spin entry, spins, and spin recovery procedures in an airplane or glider, as appropriate.
(a) Except in the case of a retest after a failure for the deficiencies stated in § 61.49(b), the examiner may either accept the logbook endorsement
or require the applicant to demonstrate competency and instructional proficiency in stall awareness, spin entry, spin, and spin recovery procedures.
(b) If FAA Form 8060-5, Notice of Disapproval of Application, was issued due to deficiencies in competency and instructional proficiency in stall awareness, spin entry, spins, and spin recovery procedures, the applicant is required to spin the aircraft and demonstrate competency and instructional proficiency in stall awareness, spin entry, spins, and spin recovery procedures on the retest.
(c) The examiner and the applicant are not required to wear parachutes for spin task demonstration during a flight instructor practical test.
(7) For first-time flight instructor applicants, both the instructor who provided all the required training and the recommending instructor making the endorsements must have met the requirements of § 61.195(h). This requirement does not apply to flight instructor applicants seeking an additional rating because those applicants are not considered first-time
flight instructor applicants.
C. Flight Proficiency Endorsement. An applicant must have received flight instruction by a flight instructor meeting the requirements of § 61.187(b) in
the subjects appropriate to the flight instructor rating sought.
(1) The applicant’s logbook must contain an endorsement from that flight instructor certifying that the applicant is competent to pass a test on those subjects.
(2) Other flight instructors not necessarily meeting those requirements may have given the applicant instruction; however, some instruction in each subject and the appropriate logbook endorsement must have been given by a flight instructor meeting the requirements of § 61.187(b).