The Gemara has the same question:
Rabbi Shimon ben Elazar says: The halakhot pertaining to a gentile
who resides in Eretz Yisrael and observes the seven Noahide mitzvot
[ger toshav] apply only during a period when the Jubilee Year is
Rav Beivai said: What is the reason? It is derived from a
verbal analogy between “well” and “well.” It is written here, with
regard to a Hebrew slave, who can be sold only when the Jubilee Year
is observed: “Because he fares well with you” (Deuteronomy 15:16). And
it is written there, with regard to a ger toshav: “Where it is well
for him; you shall not wrong him” (Deuteronomy 23:17).
It is interesting to note that although there are some opinions learn this gemara to applying to only some laws of ger toshav, according to the Rambam the statement of the gemara is absolute. However there is still an obligation to "compel" all the inhabitants of the world to accept the 7 Noahide laws as stated in Hilchos Mlachim (8:10).
Regarding keeping the ger toshav alive the Raavad agrees with the Rambam that nowadays it doesn't apply. The reason he says is that the Jubilee Year also caused Shmita. During Shmita, everything became ownerless, so it wasn't a burden to also keep the גר תושב alive. However, nowadays Shmita doesn't apply, so keeping the גר תושב is a burden on the community.
וקרוב הדבר להיות מן הטעם כי בזמן היובל היו שומטין והיה יכול להתפרנס שלא בטורח צבור ועכשיו אינו יכול