Because it’s the simplest form of mechanism for SPF, and the most understandable in my opinion.

a and mx mechanisms are just ip4/ip6 with extra resolving steps and more complex rules. For a selfhosted setup where you only have a single IP address, this is the most straightforward

As an example, try to guess which one of these SPF records will or will not pass SPF, given that the IP address sending emails is 1.2.3.1:

$ORIGIN domain.tld.
$TTL 1d

@        SOA      pluto.domain.tld. admin.domain.tld ([…])
          NS      pluto.domain.tld.
          NS      saturn.domain.tld.
          MX 10   mx.domain.tld.
           A      1.2.3.5
         TXT      "v=spf1 a -all"
         TXT      "v=spf1 mx -all"
         TXT      "v=spf1 ip4:1.2.3.1 -all"

jupiter    A      1.2.3.1
pluto      A      1.2.3.2
saturn     A      1.2.3.3
venus      A      1.2.3.4

mx     CNAME      jupiter.domain.tld.